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the penumbra was unusually pale, and the rnnbra of a decided light brown hue. Four darker openings arranged in a square were seen in the umbra, and were readily observed with a very small aperture. A very remarkable circumstance in connection with the sun's spots daring the last two months has been their extremcly light colour. The light brown tints of the umbra have been very marked, and totally different from the dark hues they usually present, while at times the penninbrœ have been so light as to be scarcely visible. In most of them, however, the nucleus (which is ordinarily so difficult to detect) has been very easily seen, as in the case of the foregoing observation. This fact proves the phenomenon seen due to the actual lightening of the spots themselves, because if it were merely an optical or atmospheric effect the whole spot would be lighter, and tho nucleus would be quite as difficult to detect as before. It is probable that these appearances may be a necessary result of the maximum of sun-spot activity/and are due (as suggested by Mr. liockyer) to the thinness of the solar envelope at the present time. This would certainly account for the light hues of the umbra? and penumbrie, and also for the frequency and blocknc&s of the nucleus. Mr. Henry Ormestier, Manchester, writes:—" On the Slst July, from iih. 15m. to Sh. Um., while looking at the sun with my Sin. refractor, I saw a beautiful large cluster of spots occupying an almost central position on the disc. It occurred to me that the umbra in the largest spot appeared more dense on the western side. I therefore determined to examine it with my .r»Jin. refractor. I did so, using a power of 180. The result was that it resolved itself into a very fine nucleus of a somewhat oval shape. After making myself sure that the above was the case, I exumined the cluster generally and was etrnck with the beautiful appearance of the brighter part of the sun's atmosphere. A very bright stream ran across the cluster in a zig-zag direction, separating the penumbra. Some parts of this stream, and particularly tho upper part, appeared brighter than others, presenting a very mottled appearance." Mr. William F. Denning, of Bristol, observed the sun with Ins 3in. refractor, July 14th, from 5h. SOm.toOh. 10m.; he noticed ninefwell-denned macule on various parts of the disc. A particularly large and interesting group of spots was visible in |the N. hemisphere. On July 22ud, at 8 p.m., a spot was observed in tho same hemisphere, which was divided by two bridges of light. He noticed that the penumbra was invaded by numerous minute lines of light, and that the linages seemed to present the appearance of running imiter. This observation was made with power 100 on a lOJia. reflector, by Browning.

The Lunak Eclipse Of July 12.—The Rev. Ralph Prowde, * of Northallerton, Yorkshire, observed this phenomenon, and has sent the following :—I observed the eclipse of the moon on the 12th, but the only thing remarkable was the great contrast of shade between the darker and brighter penumbra). I say penumbra, for I suppose the real umbra of the earth's shadow falls within the moon's orbit. The darker interior cone of shadows obscured the edge of the moon and the objects on its surface as it passed over them almost entirely, but its own edge did not seem to be nearly so regularly round as the lighter enveloping cone of shade." The Rev. S. J. Johnson, of Crediton, reports :—" I had a very favourable view of the lunar eclipse. The sky was clear at first, with a small amount of stratus near the horizon. I first caught sight of the moon at 8.41, but it was 8.49 before it got clear of the clouds. I paid particular attention to the degree of distinctien with which the eclipse portion could be Been. When about four digits were covered, I just noticed the copper tint through the telescope. I fancy this would be a little sooner than in the bist eclipse I observed (September, 1807), but in that of October 4,1865, which was only of four digits, the copper tint was very decided at the time of the greatest obscuration. When about six digits, or half the disc, was covered, the copper colour could be clearly seen with the naked eye. I could not make out any particular parts of the moon's surface until 1.35, when I noticed the mare tranquillitatie and the mare terenitatie showing with beautiful distinctness through the earth's shadow in the telescope. A few minutes after the totality was attained, I was strnck with the obscurity of tho eastern sido of the moon being so much more than I hod expected. At 9.G5 at least half of its surface was as if blotted out, eveu when seen through

the telescope, although I applied two different powers

TO'and 150°. Three of the tea» at the western side were all I could make out. Possiblv a thin coating of cirrus cloud which covored all the" sky at this time might account in some measure for the' invisibility of the moon. By 10.30 this had entirely cleared away and the Milky Way, very near the moon, was about as distinct as it usually appears on a dark clear night. At this period, being the middle of the eclipse, the upper portion of the moon was the invisible part, all those regions lying round the margin of the disc being alone to be soeu except at the vertex where the margin itself was not discernible. At 11.23 the first streak of light was breaking forth at the eastern edge. At 11.45 the red colour was nearly gone and the eclipsed part appeared of a grey colour. At 11.58 I noticed there was no trace of the Milky Wav. At 12.24 the lunar circle was complete." Mr. Oliver J. Lodge, of Hanlev, reports that " the colour of the moon daring the totality was of a most peculiar copper hue, giving very little light indoed. But during the egress of the shadow it was almost as white and silver}- as it usually is, although etill under the penumbra." Mr. Edmund Neison, of London, says :—" The colour of the eclipsed disc was during the whole time a dull yellowish olive green, both in the telescope and out, but was never dark enough to prevent many of the chief markings and craters being seen. From 10.44, when the lunar disc

was fairly above the fog banks, Arulnrehuima quite distinct as a bright crater, and even before eleven örimuldi was plainly discernible." At Bristol, Mr. William F. Denning observed the phenomenon, and remarks that even at the time of totality many of the most conspicuous objects on the disc wore distinctly lisible. The copper tint was also very evident. During a portion of the time the moon wu.-. overcast with clouds.

Venus.—Mr. Henry Ormestier, of Manchester, observed this planet with SJin. equatorial refractor on Jnly 23, at 5 a.m. •' The definition was excellent. I observed three dusky spots on the disc, one of which was of very considerable magnitude."

Satubs.—Mr. H. Michell Whitley, of Pcnartli, Truro, writes:—"Jnly 7tb, 10h. 11m., power 20S:. The ball of the planet dull yellow colour. N. equatorial ruddy belt conspicuous, and another of same colour between it and pole; polo bluish grey. Edges of disc fainter than centre. Sky in Ansa' much blacker than around planet. Crape ring across boll nearly us dark as ball's division, pale purple. Crape ring Very easy in Ansie. No line of light between it and B."

Occultation,—Mr. Joseph С Lambert, of Sleaford, witnessed the occultation of B.A.C. 5054, on July 10, and found the exact time of disappearance to be 12h. 40m. 41s. mean time.

Meteors.—Mr. J. C. Lambert "obsorved a very brilliant meteor at llh. 40m., July 21. Course from a little below ir Cassiopeia.' to f Persei. Nucleus appeared as a star of 1-5 mag., tail nearly 2' long.; colour yellowish white. Duration 2 seconds. During the time of observation, llh. to 12h. 80m, I observed no less than eleven small meteorites. The course of one of these was from i Bootis to 43 Сотев Berenices, and immediately afterward one from a little below 48 Comte Berenices to ., Bootis. Conld this have been one and the same meteor describing an arc?"

Lunak Observations.—Mr. H. Michell Whitley has made very careful observations of many interesting lunar objects, and the results he has obtainedhave been sent to Mr. W. R. Birt.


[We do not hold ourselves responsible for the opinions of our correspondents. Tho Editor respectfully requests that all communications should be drawn np as briefly as possible.]

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"I would have every one write what he knows, and as much Rb ho knows, but no more; and that not in this only, but in all other subjects: For such a person may have some particular knowledge nud experience of the nature of euch a person or such a fountain, that as to other things, knows no more than what everybody doee, and yet to keep a clutter with this little pittance of his, will undertake to write the whole body of physicks: a vice from whence great inconveniences derive their original."—Montaigne'* Ettayi.

*»* Is order to facilitate reference, correspondents when speaking of any Letter previously inserted will oblige by mentioning tho number of the Letter, us well as the page on which it appears.

THE SATELLITES OF URANUS. [257] Sir,—Sir John Herse bel gives elements according with Sir William's account. Will " Etudiant" quota Mr. Lassell's exact words? I cannot but think there is some misapprehension. In 1851-2 the determination of the place at which a Uranian satellite reached its greatest elongation would bo a matter of extreme difficulty, since the apparent path of the satellite, so long after the Uranian equinoxes, would be an ellipse of but moderate eccentricity. The difference of apparent distance when the position-angle of Uranus was 1874', aod 172J would not be great.

Kiciid. A. Proctor.


[258] Sir, — The cirenmstanco referred to by "M. L.," (see 285 and 230) would have no effect on the range, whether the ball were fired at a great or small angle with the horizon, and to whatever height we conceive the ball to rise, unless atmospheric resist anee be taken into account. If we conceive two balls, A and B, fired from a point on the equator at equal angles of elevation and with equal velocities, one due west, the other dne east, and a perpendicular from each extending throughout the motion, to the ground; then the motion of A's perpendicular along the ground will exceed the motion of B's, while the balls are rising. And, therefore, if A and В could be snddeuly stopped at tho one moment when each reached its highest point, the horizontal range of A would be found to be greater than that of B. But in the descent these conditions aro exactly reversed, and at tho moment of reaching the ground A would be as far to tho west of the point of projection as В to tho east. But if we take into account tho resistance of the air, we have

a more complicated matter to deal with. I have not time to go very carefully into the problem, which ii probably one of some difficulty; but I believe it will Ы found that the westerly ball will have slightly the greatest range,—for this reason, that the horizontal motion is greater while each projectile is rising than while it is falling, and the gain of the ball A while rising is therefore greater than the gain of the ball В while falling. But though here the chief effect (I expect) has been considered, i.e., the action of the air's resistance in modifying the shape of the trajectory, vet an effect due to another cause has not been taken into account,—I mean the fact that the upper regions of air are carried somewhat more swiftly than the lower, being at a greater distance from the earth's centre. Owiag to this, the ball (B) which is fired towards the east encounters the horizontal resistance due to its horizontal velocity, minm the difference between the motion of a point at the earth's surface, and that of a point considerably above the earth's eurface (at the equator i; while the ball (A) fired westward encounters the horizontal resistance due to the sum of these velocities. Combining this with the accepted relation between velocity and resistance, we can form an estimate of the effect of this cause in diminishing the range of the ball A as compared with that of the ball B. The balance must be struck between this effect and the former. But the only way of rcaly deciding the question would be to take the equations of motion, and at least partially integrate them. I have the equations before me, but I do not like their look by any means. Richd. A. Proctor.

THE IRIS DIAPHRAGM. [259] Sir,—In reply to "Unit" iquery 4495), I forward a sketch of the only plan known to me, having never seen any other. Of course the details of construction may be varied indefinitely to suit different pnrposes and positions. It consists of five thin metal leaves pinned on to a back plate, on which a loose ring slides by means of slots and studs, carrying also the studs that take into the slots in the tails of the five leaves. Washers must be provided nnder the pins of the leaves to make up the thickness of the slidini; ring The entire thickness will be about S-16in., and ¡tS essential that the broad end of the leaves be thin in order to close well.


The "Iris Diaphragm" may be used with great advantage in telescopes, in photography, and is especially requisite to complete the illusion of dissolving views, to which it has never yet been applied. The angular motion of the sliding ring may be effected by hand, by lever, or for slow motion by an endless screw and portion of a worm wheel. That motion extends to about 50- or 60\ Henry W, Reveley.

ALGOL. [280] Sin,—I would suggest, as that very interesting variable star, ß Persei "Algol," is convenient for observation in tho evening, that "F.R.A.S.," or other astronomical contributors, should give for the advantage of amateurs the times of the star's variation, which takes place every third day, from 2nd to 4th magnitnde. R. T.

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Г261] Sib,—Engineers generally try to get in long excentric roda, and in ordinary locomotive practice the length of tho rod is often as much as twelve times the throw of tho excentric, and seldom I believe less than ten times; but in marino screw engines and in eome paddle-wheel engines they arc compelled through want of room to nsc very short rods, and lengths of five or six times tho throw aro common.

In dosigning a link motion the first things to be considered are tho proportions of the slide-valve, and they are of conrse determined by the capacity of tho cylinder and the epecd of the piston. Suppose it is determined for a given engino to have a valve with Jin. of lap, and 4Jin. of travel, and Jin. of lead in full tkrow, the next things to be determined are the lengths of the excentric rods and the longth of the link. Say that space limits ns to a Sft. rod, and as there is no law for the length of the link any more than the exceutric rod, wo will set it down at 12in. from centre to centre of excentric rod

Sins. The " lines " of this motion should then be laid own on the drawing board in tho two positions shown in Fig. 8. Draw the main centro line С V as that which passes through tho centres of the crank shaft and valve-spindle ; lot С be the centre of the crank shaft, and round it draw a circle 4Hn. diameter for the path of the centres of the excentrics, which is equal to their throw; take diametrically opposite points F and B, at right angles to tho line С V, as the temporary positions of the centres of the forward and backward excentrics; draw two lines H and E parallel to the main centre line and at equal distances from it, and let them be the length of the link (12in.) asunder. Then take »ft.— the length of the excentric roil—in the trammel ; set one leg in centre F, and mark with 1 the point of intersection of the other leg with line H, and with 4 its intersection with line K. Then set the trammel in centre 11, and mark with 2 the intersection of the other leg with line K, and with 3 its intersection with line H. Again set one leg of the trammel in point of intersection 1, and the other leg wherever it comes on main centre line С V, and draw the arc 1, 2; do the same with point of intersection 3, and draw arc 3, 4. Theso two arcs are the true curvation of tho link, and their distance asunder is the amonnt of motion caused by the crossing of the rods : in the present case it is i in., •r Jin. on each side of the point M D, which is the

mían dMtanc« of the centre of the link from tho centre of the crank shaft, and the centre of the link will be at M D when the valve is at the middle of its etroke, covering the ports seven-eighths of an inch on each end. For distinction sake let ns call these two positions of the link the poHtion* of no Untat advance.

The valve'has seven-eighths of an inch of lap, and oneeighth of lead, giving lin. for the lineal advance of the excentrics. Draw line L A lin. from centre С and parallel to F В, and where L A intersects the circle at F' and B' there are the proper positions for the centres of the excentrics. Onco more take the trammel—still set at 3ft.—and place one log in F\ and mark point 5 where the other leg cuts line H ; do the same with point B', and mark point 6 on lino К ; draw the arc 5, 0 as before directed, and there we have the position of tho link when the crank is on the back centre at С The dotted lines show the positions of the gearing when the crank has made half a revolution, and arrived at the front centre at C", and the rods have crossed.

It will bo seen that there is a distance of 2Jin. between these two last positions of the link, although the lineal advance would in itself account for only 2in. distance, but to that we have to add tho Jin. of motion caused by the crossing of tho rods, making altogether 2}in., or Hin. on each side of the " mean distance point." But the valve covers the port only Jin., and therefore, when it is moved ljin. from that position by the link it must leave the port Jin. open, or in other words there is Jin. of lead with this valve motion when the link ie in " middle gear," and only Jin. when in "fall throw " with the end of the link opposite tho valve-spindle.

In this position, Fig. 4, the forward gear excentric rod is exactly opposite the valve spindle, which therefore derives its motion exclusively from this rod for tho time being. The apparatus now assumes tho simple character of a valve driven by a common tingle excentric and one rod. The effect of the crossing of the rods has entirely disappeared from this end, but it remains unaltered at the centre of the link, and is doubled at the other end. This is plainly shown by the " positions of no lineal advance" 1, 2, 3, 4, which are coincidental the forward gear end, jin. asunder at tho middlo, and ljin. asunder at the back gear end of the link. But that does not increase the total motion at that end, which is ever and always equal to the throw of tho excentric, notwithstanding what Mr. Harrieon says

about " that end of the link having double the vibration," and the vulve has tho exact amouutof lead (Jin.) due to its lap und the lineal advance ; but from thut position the lead gradually increases as we approach the centre, when the maximum of Jin. is attained, for tbero the whole effect of the crossing of tho rods is felt. Of course the same observations apply to the other half of the link when the rod which actuates it is placed opposite tho valve spindle.

Hitherto I have spoken only of that arrangement of link motion in which the lead is orratcr in "middlu gear" than in "full throw." 1 shall now glance at thut arrangement which producís a contrary effect on the lead, making it leu iu "middle gear." This is illustrated in Fig. 5. In this figure all dimensions aru the same as in Figs. 8 and 4, and the same letters of reference ure common to both, the only diiTerenco being in the disposition of tho rods, which is v, ntrary to that of 3 and 4, for in this the buck gear rod is attached to the upper end of the link and tho fore gear rod to the lower end. The vibration at tho emit of tho link is still of course equal to the tlirow of the excentric, and the amonnt of motion transmitted to the vulve tu /till throw the same as in Figs. 8 and 4 ; but the motion at the centre of the link is considerably reduced, for it is now only ljin. or 11-lCths on each side of the "mean distance point;" bat tho valve has Jin. of lap, and wonld require to move that much on each side of its middle position before it would begin to uncover the port at all; but the link can only move ll-16ths on each side of the "mean distance point," therefore in this arrangement of the motion, when the valve has Jin of lead at the end of the link or in " full throw," tí i« actually 3-iethe blindât the centre, or in " middle gear," and in order to get ¿th of lead in that position, we must canse the centre of the link to vibratt' 2in. (think of that Mr. Harrison), and to do that tho "lineal advance " of the excentrics must be increased from lin. as at present to about 1 D-lCin., shown in dot lines, and then the lead of the valve when in full throw will be about 7-ltiin.

Now what is the reason of all this? Why is it that in Figs. 8 and 4 the vibration of tho link at the centTM is so much greater than it is in Fig. S, the lineal advance and every other dimension remaining equal 7 The crossing of the rods produces a similar effect in both cases, viz., Jin., as is shown by the "positions of no lineal advance " 1,2, 3, 4, and does not account for any of the difference, and we must therefore seek it elsewhere, and that too without going into the geometry or trigonometry of the case, for which I know the majority of your practical readers have no relish (neither have I).

Fig. 6, is just a portion of Fig. 3, extracted and laid down separately for sake of greater distinctness. С and C" show tho crank on the two dead eentres; F' and F are the corresponding positions of the centre of the forward gear exceutric, tho rod of which is carried to the upper end of the link; and between these two positions of the end of the excentric rod there is a space of 2 J in.

Now look at Fig. 7. It is a portion of Fig. 5, showing the forward gear rod attached to the loirerend of the link. In this case there is a space of only 1 Jin. between the two positions of the end of the rod, and ns the back gear rods in both of these cases perform the same as the forward gear rods, there is no necessity to show them or the links, and the figures are plainer without them.

The alteration of the lead of the valve is a grave defect of the link motion,—the ihiftina link motion, I mean, such as we have been considering hitherto; but the iditioftary ¡in* is freo from that defect. The "shifting link " is so called becanse in the act of reversing tho engine or altering the grade of expansion, the link is moved up or down by the reversing lever or wheel, as the case may be ¡ but the "stationary link," as its name implies, is a fixture, and is suspended or sustained from some fixed point, and has no motion except that imparted bv the excentrics. Tho reversing lever has no connection with this link, but tho engine is reversed or expansion altered by means of a rod called tho "valve spindle connecting rod " one end of which is jointed to the head of the valve spindle, and the other end carries the elide block, which tits in the slot of tho link. The reversing lever is connected with this "valve spindle connecting rod " by any suitable means, so that the attendant can plnco the slide block to work in any part of the link, either for reversing or expansive working.

The "stationary link," as regards its curvature, is placed tho reverse way to the " shifting link," for it is set with its concave side towards the valve spindle, and the radius of its curvature is the length of the " valve spindle connecting rod," bnt in the shifting link motion the radius of the link is the length of the excentric rod.

The stationary link motion is a much more elegant and scientific piece of work than the shifting link, and it possesses the great advantage of preserving the lead unaltered in all positions; but it has not come into general use, owing to the great length required for its proper development,—a length altogether out of the question in screw boat engines or locomotives.

As I intend to have more to say on this subject, with your permission, in future letters, I shall here bring it to a conclusion for the present, hoping that Mr. Harrison may benefit by what I have written, and acquire therefrom some correct knowledge of tho construction and working of the "link motion," in place of tho orroneous ideas, the absurd orotchets, and tho fantastic notions with which his head was so foil when he wrote his notable letters iu No. 272, page 352, and in No. 280, page 471.

James Baskebvillb.

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1/JOiij Sir,—I undoubtedly invited discussion, not about и gas companies, bat "how to reduce our gas bills," and "improve our light);" nor do I complain of fair and honest criticism. "Sigma" is far too able ft chemist for mo to attempt, if I wished, to argue the point ho raises, even if you would allow your valuable space being so takeu up. lint I must totally differ аз to his assertion that "it is not even true that the illuminating power is reduced by the alleged removal of illuminating hydrocarbons from tho gas." This is a bold assertion for him to make, and is also ж contradiction in itself. He admits, farther on, that ,L some of the light-giving properties of the gM are removed," because the gas "cannot be purified without doing so." Other chemists differ from "Sigma" on this point: be it so or not, my words are continued by " Sigma " himself, and while other towns are supplied with far richer gas, without difficulty being caused from deposits in the mains, it is idle to tell us that it can't be done in London and other localities, though I hope "Sigma" will believe me to have sufficient common sense not to expect that all inland towns can be economically supplied v. it h such rich gas as Aberdeen and Glasgow.

Your readers can also now judge as to " Signia's" remarks about the price of London gas; he first told ns it was the fault of the consumers ; and now he admits that •nine companies pay u full dividend of from 7 to 10 per cent, and, nota bonus us I culled it, but "arrears allowed by law." In other words, tho price is kept up to enable the said companies to divide this excess of profit, to cover their previous mismanagement and their own '• ruinous competitions 11"

As "Sigma" admits Professor Frankland is not а "gas quack," perhaps you will kindly allow space for the accompanying notice of that gentleman's lecture on coal-gas, before the Royal Institution in lb07, as it will show your readers and " Sigma " that what has been said by me is not all "senseless gabble."

Professor Frankland said that he had just had the illuminating power of the gas supplied to different large towns tested by the standard sperm candles, and then held written and signed certificates in his possession of tho results as follows :—Berlin, 15-5 caudles; Paris, 123; Loudon, 12*1;'Vieuna, B*0; Edinburgh, ¿8*0; Manchester, 22*0; Liverpool, 22*0; Glasgow, 98*0j Aberdeen, '¿ÖV; Greenock, 28*5; Hawick, 80-0; Inverness, 250; Paitdey, 30*3; Carlisle, 160; Birmingham, 150. Thus the gas supplied to Edinburgh and Glasgow gives more than twice the light of the gas provided for London. The above shows the average light given by the gas furnished in London; but, in particular instances, it only equals nine candles. The gas of London also, he stated, is richer than it ought to be iu sulphur compounds, and in burning gives off too much poisonous sulphurous acid aud other gaseous vapours injurious to health and property. London gas is now worse than it was many years ago, although the methods of manufacture have been cheapened by the discoveries of science, all new inventions in this direction having been eagerly taken up by the gas companies, who, so far as is known, have not adopted u single invention which would benefit the consumer. As the gas is damaging to works of art and beauty, the public should, by paying а fair price for it, at least have it of good quality. He concluded by saying, "it should have an illuminating power of twenty candles, below which no gas is lit for household use."

It is indeed much to be regretted that these scientific and interesting subjects :tre not discussed in a more courteous and liberal spirit, without indulging in personalities and dogmatic MMttioM and iniaquulut-una üif each other's words and intentions.

Another correspondent takes objection to tho experimental result I gave of the improved light obtained by the very crude apparatus described in No. 868, of the 8th April, but I must explain that such was given simply to show what was the comparative result of the process with the same burners, pressure, Ac., aud not to prove the absolute standard of light obtainable. When that experiment was made, I was living where we were supplied with wretched gas, at Cs. per 1,000ft., which first led me investigate the matter and devise a remedy. Where I am now living we have very fair gas, but I believe there is no law fixing the standard of light to be supplied by rural or provincial gas companies, so that ** Sigmu's " remarks do not apply to such cases.

I must observe that what has been eaid by "Sigma" and also by "L. M." (p. 400, No. 60), abont street lamps, does not apply to our domeetic gas consumption. "Street lamps" require a very small and imperfect carburetter to be fixed to each lamp, and not (as in a house) only one apparatus for all the burners. Dr. Letheby's official report on the experiment distinctly states that tho light was at first most excellent, but gradually fell off, and caused great complaints, in consequence of which the attempt failed, as I stated in page 354.

To prove that this is not the case with such apparatus as I havo recommended, and which can be now obtained at moderate prices, I subjoin the results of the last year's operation of the one I have used for years, which I designed and fitted up for my own amusement and most decided advantage, for it only cost about 6s., and saves me fully 25s. a year, besides the advantages resulting in better air and less destruction to property. Not (as "Lucidus" wishes to make out that I state) from any power of "purifying" in the apparatus, but simply suchas is due to the reduced quantity of gas consumed to obtain the required power of light. I filled the carburetter with 1J gallons of carboline, sp. gravity *775, April, I860, and found tho power of a small burner (consuming 2ft. of gas per hour, at pressure of 8-lOths) to equal to 6 5 candles.

On 1st August, 1H70, tho carburetter was emptied, after being in constant use and untouched for sixteen mouths; four pints of fluid remained, sp. gravity •785, which gave a power of light, with tho «a«w burner and consumption per hour, equal to 5*9 candles. The carburetter was then refilled, including the residuum above mentioned, when tho power of tho light was found (all exactly as before) to be equal to 6*3 candles, an interval of twenty-four hours being allowed between the last experiments.

The total of gas consumed iu sixteen months was 13,000ft. The number of burners in daily use was three, and occasionally six, with two small gas stoves, by Phillips, in hall and bod-room, iu winter nights burning 5ft. per hour of gas, which was not carbure tted.

Your readers can therefore judge roughly from the above what advantage there is to be obtained in the use of a good carburetter, and if it is wished I shall be happy to give a drawing and description of the one I use. which is tiüí a patent; and as I have given yon my name, rank, and address, I feel confident you will not impute any improper motive iu all I have written, which I thank you heartily for allowing to appear at such length in your really most valuable journal, now a marvel of cheapness and interest. С D. C.


1263] Sin,—As I see no ono has yet replied to the inquiry of your correspondent about Irish moss, I beg to inform him that it is a kind of sea-weed found on several parts of the coast of Ireland, where it is colled carrigeen, and it can be obtained at almost any respectable druggist's under its English name. When dried it is picked over, and the finer and clearer parts are kept for medical use, while the darker and coarser are used for feeding pigs, for which purpose it is largely used in many places, boiled up in their ordinary food. Its qualities are somewhat similar to those of Iceland moss and isinglass, but it has the advantage over the former of being nearly tasteless, and over the latter of having a specific action on the lungs, which makes it nearly if not quite' equal in value to cod-liver oil. I have known eeveral cases where it alone has effected a cure after tho patieut had been supposed to be in hopeless decline. The manner of using it is as follows :—Take about as much of the dry moss as will fill a common teacup, and steep it overnight in cold water, which removes most of the marine taste which belongs to it. In the morning boil this just ten minutes in a quart of milk, season with any flavour that is agreeable, and when cold it will bo found a strong jelly, like blancmange, for which, if carefully made, it may be substituted. It may be made with water as well as with milk for variety, and is particularly brnoflcial if taken in the morning {melted), about a tcacupful at a time. Its action in some cases, especially when there is hemorrhage from tho lungs, is wonderful. The ordinary price is about 3d. or id. an ounce, which, as it is very light, is not expensive; but it is at least a dozen times what it ought to be. A few years since, when I was living at Leeds, I was able to procure it at one shop for 4d. per lb., while the druggists were selling it of the same quality at the same price per ounce. If your correspondent who wishes to find я substitute for cod-liver oil will try this I have little doubt he will derive benefit from it. G. Bush.


|264 J Sib,—While I agree with the "Harmonious Blacksmith" as to the probable enlargement of the English Mechanic at no late period, I differ from him as to the increase in prie, for this reason, that many a subscriber who cheerfully pays his twopence weekly now, would, even for the same paper though much larger. begrudge to give the extra penny; and I therefore beg to endorse the able and sensible remarks of your new correspondent " Hopeful," as to an increased circulation, which I am sanguine enough to think could easily be carried into effect were each of yonr present subscribers to use but a little exertion, and exhibit your most valuable journal to their friends, fellow-workmen, and acquaintances; for it needs no puffing, and only requires to be Been und read to be appreciated. Yonr efforts in the cause of science and information on general subjects are deserving of the highest praise, and it is only but right that your noble venture should prove a pecuniary success.

J. B. Cockburn.


[265] Sir,—Permit me to inform your correspondent "Raymond," (4531, p. 527), that the plan he proposes for combining an object-glass would be quite erroneous. A plano-convex lens of flint, in combination with a crown lens of the same form, would only tend to increase, and not correct, the chromatic dispersion, while the spherical aberration would remain almost the same. The theory of the achromatic objectglass consists in correcting the dispersive power of the convex crown lens, by using a concave lens of higher dispersive power but shallower curvature; and so the opposite tendency of one counteracts the opposite tendency of the other. If "Raymond" seriously thinks of attempting an achromatic О. O., he had better examino one first, and веек to discover the reason for working tho four surfaces to different curvatures. Plano-convex lenses, when large, are objectionable on account of the greater difficulty of working a plane than a carved surface. Again, the process of parabolizing is required only /or rejtectore;

the surfaces of an object-glass are all supposed to be spherical, the spherical aberration being removed by the concave form of the flint lens. As to the suitability of my machine—to which "Raymond" refers—for working the lenses of an object-glass there is not the slightest doubt, and any object-glass can be tested by the same process that I have described for specula, the only difference in method being that the test-bar should merely be a narrow strip of paper made to adhere to the surface of the object-glass while testing. As to the query of A. White, on the same page, I believe the druwback he would find would be a want of good definition, which would be equally as noticeable in his proposal as in the "Herschelian" form of telescope, and which, when using high powers, would be simply intolerable; such an arrangement might, however, be made ju»t tolerable by making the focal length of the speculum unusually long for its aperture and nsing only moderate powers. The mischief arise*, of course, from the obliquity of the cone of reflected rays with respect to the incident rays, the axial lines of which should exactly coincide with each other to secure the best definition; this necessitates the central position of the plane, and this form, commonly called the "Newtonian," undoubtedly maintains its superiority against all others. W. Purkiss.


[266] Sir,—On the terrestrial globes, besides the map of the world, there is placed a figure extending an equal distance on each sideof the equator, and reaching ts far as 23) - latitude, to this figure has been given the name of Anolemma, though it is, in fact, nothing more than the namee of the twelve mouths with the number of days in each month, and so arranged as to correspond with the sun's place on each side of the equinoctial. Thus, for instance, the 21st March and the 20th Sept. are placed on the equinoctial line, and come under that part of the brass meridian which is marked 0, and this serves to show that on those two days of the year the sun has no declination. At the northern extremity of the Analemnia is June 21st, and this is made to correspond with 23f" North Declination, and in like manner the southern extremity is marked December 21st, and comes under 23 j"' S.D. The intermediate divisions or gradations being made to correspond with the different declinations of the sun. Now this is a most ingenious and easy method of giving at a glance the explanation of many of the most important phenomena in mathematical geography, and the uses of this figure are varied and most important. Numerous problems may be answered instantaneously, if we only know the principle on which the Anolemma is constructed; as, for instance, the sun's declination, in what latitude he is vertical aud on what days, the time of his rising and setting, the duration of twilight, and his meridian altitude ; so that the Analemma is one of the most useful figures on the globe. In the present instance we shall see its utility in finding the length of tho longest days in the frigid zones. Geographers have found it convenient to divide this part into spaces corresponding with the length of time the sun is abore the horizon—as, for instance, where the longest day is one month loug, where it is two months long, and so on, until it reaches six months long. Now as the sun can shine over half the «lobo only at once, and as the earth rotates at the poles, some of the places in the frigid zone, in the coarse of the year, will never go below the horizon at the North Pole six months, 234' from it one month, and gradually increasing in the intermediate spaces till the pole is reached. When the sun has 6° declination, those within ГГ of the pole will have constant day ; when 10 declination, those within 10 of the pole will" have constant day; consequently the nearer any place is to the pole the longer will the sun be above the horizon. To find, then, tho number of days during which this is the case, we have only to look on the Analemma, corresponding with the degree of the sun's declination and the co-latitude, which, in this example, will be the some, and we shall find what we require. When his declination is high, the ecliptic and the tropic of Cancer on the globe nearly coincide, so that the space where the day is one mouth long is within a narrow strip measuring only 40', bnt the second strip is wider, being 2" 15', the strips gradually increasing in width until the last, which is 7J wide. In Nova Zembla, whose latitude extends from 69° to 78P, the inhabitants of the most northernly part will begin to see the i*on, nut taking any account of refraction, when his declination reaches their co-latitude, that is 123, andlooking on the Analemma, underneath 12°, we shall find thej will soe him from April 19th to August 28th, or lttl days; whereas the southern extremity, whose latitude is 69 , will begin to see the sun above the horizon without setting, when his declination is 21J. As, therefore, during six months of the year these stripe or belt* have a length of sunlight varying from one month to eix months, so when the sun is in the opposite declination they must have their nights varying in like manner. But there is this to be considered with reference to the absence of the enn, that it cannot be actually called night, foras twilight lasts until the sun gets 18° below the horizon, and refraction also raises a heavenly body when near the horizon, it will be necessary to see how long it takes the sun to reach lb below the horizon, and then we shall find that, although deprived of the sun's rays, these places are not without some compensation for his absence—for twilight, refraction, the moon, aud tho aurora borealis will do their bost to impart some kind of cheer to the inhabitants of these benighted regions. When, then, the sun reaches 18" south declination twilight will cease at the N.P., but this doclinanation he will not reach till about the middle of November, and will reach it again before the end of January, and during interval of two mouths the ijmi.ii must necessarily be above the horizon Borne part of the time, and if it affords no warmth it will at least afford light. The intermediate strips or belts, however, will never be totally deprived of twilight, because only those placea within 5¿J of the polo will be in this state. Again, in the temperate zones the boundary line where places can have uo twilight all night is 4*£' north and south latitudes, and this is ascertained in this way :—As the greatest declination is 23J , if we add to this IS , the extent of twilight, we shall have 414° M the distance from the pole of all places which enjoy this boon in the summer time; consequently, by subtracting this from У0 ,we shall have


;->' л- tin- parallel where twilight all night ceases— that is, only those places whose latitudes exceed Quebec in the western hemisphere, or Paris and Vienna in the eastern, can have twilight all night. With respect to the south polar regions, the poets of antiquity (see "Georgien," Book 1. line 244, &c.) could offer conjectures only, assigning to them the abodes of the infernáis and the presence of perpetual night; but modern science can offer reasons why the south polar regions should be enlightened during their night in the same manner as their antipodes, and regards such assertions as arising from the defective knowledge of the ancients on these points, and as embellishments for

their tine poetry.

T. S. H.


[267 J Sir,—Boyle's quotations of certain observations of a small lucid cloud near the zenith of Toulouse, between eleven and twelve o'clock p.m., on a clear evening in August, is deficient of date, so that the most important element of an explanation, viz., tho age and position of the moon, is wanting. The moon might have been so near the horizon as to illuminate vapour in the higher regiouof the atmosphere, while a little unnoticed bank of cloud, about the point of her rising, would give the general appearance of a moonless night. With your kind permission I will here place on record my own observations of a similar phenomenon at nine o'clock p.m., on 25th Oct., löÖO, in latitude 55" 46' North, longitude 4" 0' West, the moon in the 21st day of her age, and declination 20 '53' North, being E.N.E., near the horizon, but above it, and partially obscured by clouds, giving the appearance of a moonless sky. The sky had the general appearance of being cloudless, as the stars shone tolerably bright, except here and there near the horizon, which was irregular in its outline, partly from clouds and partly from undulations of the ground. On going out of doors I felt as though I was eye-witness to some supernatural phenomenon. A zone of light, stretching across the sky from near the east to near the west, and passing a very few degrees south of the zenith, met my eye. Its form was that of a rainbow, but as there were no rain-clouds visible and the sun far below the horizon, and as the bow was without prismatic colours, its origin could be in no way connected with rainfall. Its outline was as distinct as that of any rainbow could possibly be, and it had none of those twinkling motions which are characteristic of the aurora. How long it had been formed before I saw it I cannot вау, but it continued fully half an hour quite distinct, and gradually faded away. I concluded then, and I am of the same opinion still, that it was the moon's light, reflected from vapor in the upper regions, so disposed in the atmosphere as to present to my eye the form of a semicircular bow of light in a sky only faintly illuminated by the moon.

J. Steel.


[208] Sir,—I should be much obliged by a reply to the following :—

Horse Power.—1. Given a vertical semi-portable Йге-tube boiler, with internal fire-box and vertical firetubes, Mt. high, 1ft. »in. in diameter ; firebox lit. 5in. high (between furnace bars and crown of fire-box), by lft. diameter, 7 fire tubes Mt. Hin. long by 2¿in. outside diameter; water in boiler lOin. deep over crown of fire-box and surrounding vertical sides of same, giving in all, including surface of tubes above water line, a total surface exposed to fire and heated gases of about 14 square feet. Draught by a chimney 15ft. 'high. Feed supplied to boiler at about 50'. Cools of good quality. Required :—

(a) Quantity of steam of 601bs. pressure (75 from a

vacuum) obtainable from this boiler per hour t {b) If the tube surface above water line is as effective as that below in the way of producing steam У And, (c) If a greater quantity of steam would be made and how much more by raising the water line to I5in. abovo crown of fire-box? 2. Required also to know, if for this quantity of steam (sub. 1 a) anon-condensing engine of 3j|in. diameter of cylinder and 7in. stroke, with common slide valve cutting off at (say) fta stroke, is suitable ; and what would be the proper velocity of piston, or number of single strokes per minute, in order to correspond with steam supply from boiler (as above); and what would be the tluoretic, and what the real or effective horse power thu ■- developed,—presuming upon a general good finish of all working parts.

I think practical replies to practical inquiries of this sort would be of great service to many seeking information through your wide-spread and highly-instructive journal, and hope, therefore, they will be forthcoming from those of your corre s pond cuts so well able to deal with investigations of tin kind.

A. W. E.

ISOMETRICAL. [260] Sir,—I wish to draw the attention of "Isoraetros"to the circumstance that your engraver has reduced my drawing (p. 524) to make it fit your columns, and curiously enough just sufficiently to make the line A О in Fig. 2 measure lin., whereas tho projection of that line—viz., Л О in Fig. 1—is described in tho text as liu. long, which, without this explanation, would make it appear that I had overlooked his requirements under the N. B. in his query on p. 47«.

J. K. P.


[270] Sir,—Will some of my brother readers kindly advise me in the following difficulty ?—Amongst several of the same class, and of like and smaller sizes, I have a vertical semi-portable boiler, constructed on the " Field " principle, of the following dimensions :— Shell, llft.2in. high by 6ft. 7iu. outside diameter; firebox (with water space all around of 6in.), 6ft. 2in. high by 5ft. 7iu. diameter; 40 Field tubes 3ft. Tin. long, and 107 do. Wt. Ill in. long, 34, in. outside diameter; total heating surface, 6914 square feet; nominal power of boiler, 50 horses; working steam pressure, 751b. per square inch; depth of water over firebox crown, lain.; steam space, 3ft. 6in. high; smoke uptake pipe passing vertically up through steam space; draught very brisk; steam to engine takeu off close to top of boiler by half-circle 4in. pipe, having long slots on upper half of surface about Ain. broad by 2in. long each; feed eaters cold near bottom of boiler, and is seemingly pure and quite clear.

Now, although other boilers of like construction and proportions work satisfactorily, this one primes so very badly that, with otherwise efficient feed pump full on, it is impossible to keep the proper level of water in the boiler, besides which tho safety of the engine is endangered by the presence of water in the cylinder. I want to know what measures should be taken to prevent, or, at least, to reduce, the excessive priming in this instance, and an explanation of the probable canst)—taking into consideration that in other similar cases it is not found to occur. Can it be owing to the possible unfitness of feed water, and would heating same to near boilingpoint before using as feed affect the case? Oris steam made too rapidly, or steam space too small; or is, finally, the method described of taking off the steam unsuitable and open to improvement? Early advice on this to me important subject would much oblige. A Working Engineer.

P.S.—In my last query, No. 4513,I note mistakes on the part of the printer. The algebraical signs of formula No. 1 should be explained thus:—n the theo* retie II.f. (i.г., horse power) ; / the area of piston in square inches. Then, again, further on, R is used instead of H.V. (horse power). In query 8, H.P. (horse power) is also to be understood by It, wrongly printed, and the words within the first brackets of this query should be (high and low pressure). Then, again, the words " as indicated by an open mercurial " wants the the concluding word "gauge" and the right-hand bracket. In tho place of "cylinder respectively," it should be read " cylinders respectively."


[271J Sir,—Being profoundly ignorant of the nature of light—vide Mr. Proctor's letter, p. 347—perhaps he will kindly explain the cause of two curious photographic phenomena. If I tako a sensitized collodion plate, and cover it with orange-yellow glass, exposure to tho action of strong light has no effect. But if it is previously impressed with a latent image, then such an exposure not only destroys the impression, but tho plate is fogged all over. Again, if I take four pictures, one after the other, on different portions of the same plate, I find the last exposed requires nearly double the time of the first.

As Mr. Proctor thoroughly understands this subject, I shall esteem it a favour if he will enlighten my ignorance of the cause of these effects. With reference to the zodiacal light, I fancied there was another view, in addition to those described by Mr. Proctor, which shows it to be a phenomenon pertaining to our own atmosphere. Being so densely ignorant, my ideas are, however, worthy of no consideration. T. A.


[272] Sir,—Obviously the aim of all thoughtful men at the present moment should be to keep tho subject of emigration KS much as possible in the background. The disturbances on the Continent have created work for the unemployed. Our arsenals and dockyards again team with life and activity; the harvest requires labourers; the drain of German and French subjects from our shores causes vacancies in our warehouses which most, if not permanently, at least temporarily be filled up. Being impressed with these ideas, my intention was to refrain, for tho present at least, from giving vent to thoughts upon the subject. However, the letter of " D. G." raises questions which deserve consideration. Candidly, I cannot commend the author of that letter for his thoughtfulness or knowledge of his subject. How am I to understand such phrases as " I would advise none to go who have not had some experience as practical farmers, or," Ac, and " this would in no way exclude such mechanics as smiths," ate. If the advice given were followed, viz., that л. farmer, smith, and carpenter should emigrate in batches, I think emigration would soon be stopped

from the want of practical farmers. Again, as a rule, it is not the last-mentioued class who emigrate, or who benefit to any great oxtent by emigrating. Further, pausing at his declamation against Cauadiau seasons and hardships, is it not a fact that the American winter and summer .. are also extremes? Is Western America a panacea for all the evils under the euu? The prairie, I suppose, offers no hardships, it is well intersected by good roads, comfortable houses may there be found in medium quantities, water may at all times be obtained, the Red Indian never interferes with the pioneer; in fact, such a beautiful picture does "D. G." infer, that one would suppose the only necessary act for a poor mechanic to do in order to gain comfort and competency ia to accept advice, gratuitously and Ben. tcntiously given, but, unfortunately, lacking the most essential quality of correctness. How does it happen, if American railway prospectuses are so genuine, that there exists so much fear regarding the flowery sen- teuces they contain, and so many who state they have been deceived by them?

Let not the emigrant be deceived, whether he visits Canada or America, and both possess advantages; he will have to encounter certain hardships, and it is extremely doubtful whether Canada, even with its toilsome wood-cutting, does not present a better sphere for his labours than Western America (at present). " D.O.' * certainly is extremely vague in the following sentence: —" He can see the hinds himself and make his bargain, then, if he is mtisfted, let him send passage-tickets to his friends." Would it not be best to make the bargain after being satisfied with the land, лс, for whether satisfied or not, after the bargain is concluded he can make no further alteration? Although I feel inclined to criticise this extraordinary letter sentence by sentence, showing the thoughtlessness of its author, I must consider your space. Then, lastly, as to the period of departure. The horrible monster of "hardship " crop» up again, and the result is, worthless advice. The emigrant's policy must be to leave these shores, so that he may be enabled to arrive at his destination during the busy season, whenever that may happen to come, be it spring, summer, or winter. If he desires to succeed the hardships he encounters will make him more energetic.

With regard to the justness of the Canadian Government, the countries open to emigrants, the warning against sailing vessels, I cordially agree with " D. G." In conclusion, if "D. G." had bestowed a little more thought upon his subject, and scorned partiality for the American prairie, giving its drawbacks as successfully as he does those of Canada, his letter would have been pronounced excellent; but holding the view as I do regarding the enormous responsibility resting upon any one who writes upon this subject, I disagree with the remarks made as stated above.

I have seen no bettor information than that contained in the Government pamphlet, entitled "Information for Emigrants to the British Colonies," M.


["F.R.G.S." would have increased the value of his letter if he had mentioned the number and page of "D.G.'s " letter.—Ed.]


[273] Sir,—I have read, with some surprise and considerable amusement, a letter from Mr. R. G. Bennett, in your issue of the 12th hist., in which he asserts the bicycle to be a mere toy, and useless for all practical purposes. This opinion he alleges to be derived from twelve months' experience of the machine, during which time he has not attained an average speed of more than four miles an hour, " using force sufilcicu t to thoroughly exhaust himself on a twenty- mil о journey." That Mr. Bennett's statement is true as regards himself, I am, of course, bound to allow; but that it represents the general experience of bicycle riders, I most emphatically deny. That he has failed in accomplishing in twelve months what ninety-nine persons out of a hundred could do in three weeks, I can quite believe, ; but I am bound to protest against his holding up his own lamentable failure as a scarecrow to intimidate others from learning the machine.

Tho bicycle club to which I belong always travels at tho average of eight miles an hour for the whole journey. When the road is down hill, we frequently run, for some two miles, at the speed of twelve to fifteen miles per hour ; up hill, we work at from four to six miles an hour; on level roads at seven to eight, or even nine miles an hour if the ground is smooth. If I go out with only the single companion, I travel faster than this, as the delays incidental to a large number riding together do not occur.

Mr. Bennett calls to his support the testimony of a correspondent, who, he states, " proved," some time since, by an "elaborate calculation," that the force necessary for walking a distance of five miles, if put into use on the machine, would only carry three miles, or little over Uco-thirds. I have no doubt that a person who "proved " three miles to be a little over two-thirds of five, would be capable of proving anything; and in any matter of theory, I should certainly beat a retreat from во unscrupulous an arithmetician. But I should be most happy to afford Mr. Bennett the opportunity of practically testing the "elaborate calculation' before mentioned, by matching my bicycle against his legs for B day's journey. I may mention for his comfort, that I could travel sixty niîïee in n day with the greatest ease (I have ridden forty miles after five o'clock in the afternoon); and as I should have to work as hard to get three miles as he has to do five [vide the "elaborate calculation ">, it follows, of course, that to prove this theory he would require to walk one hundft mile» a day, I hope he can. I may add that I am by ПО means a first-rate rider {compared with others in the club to which I belong), nor am I either physically or constitutionally strong; and again, I have not had anything like twelve months' practice.

I cannot, of course, account for Mr. Bennett's illsuccess in utilizing the bicycle, and the consequent striking difference between his experience and mine, no more than I can account for the fact that I am unable to swim a dozen yards, though I have been learning for years, and go into the water a hundred times in a season, while others acquire the art almost instinctively. Hut this is no reason why I should Bet down swimming as a useless and unprofitable pursuit, or assert that no one can swim a great distance because I cannot myself get the length of a London bath. It is unite sufficient for me to know that most persons can learn swimming with ease, and that my failure is entirely exceptional, and of course owing to my own dulness. I would have Mr. R. G. Bennett take to himself the same comfort. W. E. Maverly.

[274] Sir,—Your correspondent R. G. Bennett (p. 494) is wrong in supposing that I said it would be better on a long journey to carry the bicycle. I never made, nor should I ever think of making, such an absurd statement. I can, however, bring the testimouy of a year and a half's bicycle riding to bear ont the general tenor of his remarks, which are, nevertheless, in my opinion, a little too condemnatory of the machine. As Mr. B.'s letter is sure to evoke answers from lovers of the bicycle, I trust you will allow me to make a few remarks on the snbject in order that your readers may hear both sides of the question from those who have had real experience in the matter.

Every one will admit that the best means of locomotion under the greatest diversity of cireum stances. such for instance as the crossing of rough country, is to use the legs with which nature has provided us, supposing of course that the motive power is supplied by the traveller 1 im :elf. Our natural powers of locomotion are intended to be, as they in realitv a-e, perfect in their adaptability to diversified circumstinces. It is under partici I trcircumstances that art steps in and for the time being, supersedes nature. Thus a locomotive carries us easily and quickly over an iron road, on which a man could not walk any quicker than on a grass field—the particular circumstances in the present case being of course the smoothness and hardness of the road. If the velocipede question be viewed in this light, the whole matter may be summed up in a few words. Presuming that we have good machines (and I think they are sufficiently good), are the particular circumstances of the case, or, in other words, is the quality of our roads such as to give bicycle riding the advantage over walking to the majority of people? I maintain that the true answer to this question is not to be looked for from those few athletic individuals who, if the bicycle was twice as hard to work as it is, would still go tearing about to the risk of their necks ; neither ought we to accept the testimony of those who naturally speak in favonr of the machine, because they happen to live where the roads are as smooth аз a board. The real answer to the bicycle question can only be given by the general public, and their opinion can only he measured by the use which we see can be made of the machine. It is now about a year since I wrote and predicted the decline of the bicycle fever, and I think we need only look about ns to see that it is already taking place. For my own part, I now only see one or two bicycles where I used formerly to see a score, and my observations extend over a pretty considerable area. The fart is, the machine has utterly failed to establish its utility either, as Mr. Bennett says, for practical purposes, or for economising force. On the first appearance of the bicycle in England, it was supposed to be everything that one conld wish. It was to be ridden at the rate of 8, 10, or 12 miles an hour, and was, in fact, to go almost "by itself." The experience of a conple of years has, ï think, dispelled the illusion, and the instances in which the bicycle is pnt to any really practical use, such for instance, asa business man travelling to and from his office, ora postman earning his bags, are remarkably rare. The machine is almost exclusively used either by those who are young and strong, and who find pleasure and benefit iu active muscular exertion, or by those who merely use it for the sake of practising *' fancy" riding, and for showing off their skill in such feats as standing on the saddle.

If any one buys a bicycle with the hope of, with case and comfort, getting any practical use out of it on our roads, just as they happen to come, I am afraid he will be doomed to disappointment, and will be like many others, who, buoyed up with the same hopes, have made a like investment of hard-earned money, and would only be too glad to get it bock again into their pockets. I do not attribute the failure of the bicycle to the machine itself, though there is ample room for improvement on this point, but to the state of our roads. Let the road surveyors lay us down an asphalte path from town to town, and Ï will undertake to say that the use of the bicycle will be as common as that of a horse and trap.

With regard to the rate of speed at which a bicycle can be driven, I qnite agree with Mr. B. that some of your correspondents speak in a very random way. They talk, for instance, of " riding ten miles an hour." Do they mean that they can really keep up that speed for any distance, say for ten or twenty miles; or onlv that they can for a short distance ride ni the rate of ten miles an hour? I fancy the latter is the real state of the ease. Very few riders can, I am sure, keep up a higher rate of speed than six or seven miles an hour

f" г any distance. I have frequently travelled eleven mo isured miles, containing a fair amount of up and down hill, in an hour and a .quarter, and from my own experience I am perfectly certain that the same rate of speed could only be continued by a practised athlete. If every man in the kingdom learnt to ride a bicycle, I very much doubt whether the average rate of travelling would exceed four or five miles an hour. I am quite aware that Messrs. Jones, Brown, and Robinson have occasionally gone 100 miles in a day, and have, moreover, appeared at their desks the next morning as if nothing had happened—their spirits, no doubt, being kept up by the pleasing thoughts of eeeing themselves immortalized iu the papers; bat performances of this kiud may very well be classed with such feats as walking 1,000 miles in 1,000 hours. A sensible man would undertake neither the one nor the other, and neither the one nor the other could be performed without the expenditure of a very considerable amonnt of vital energy, which would require a great deal of rest and food for its proper restoration.

It the machine is good, the road very smooth, the wind not in the face, and the rider strong and healthy, and fond of muscular exertion, he may, with ease and comfort, get some substantial use out of the bicycle; but just in the same proportion as one or more cf these favourable conditions fail, in just the same proportion does bicycle riding become a и toil of a pleasure ;" and it is the improbability—I may almost say the impossibility—of making these favourable conditions universal, tbut renders impossible tho universal adoption of the bicycle. W.


[275] Sir,—As requested I herewith Bond you a sketch of Mordan's self-centering wire chuck. I was told twenty-five years ago that Mordau received for it the silver medal of the Society of Arts, and I made my first one twenty years ago entirely from word of mouth description, never having up to the present day seen even a sketch of it. Although I am acquainted with some of the best of workmen, I have never found one that had seen any but mine, and so shall be pleased to hear if any of your correspondents have heard of it. The first one I made had not the plain part at С to guide the cap and keep it true, it is a great improvement.

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[276] Sir,—I have read with great pleasure some articles by " Sigma " in your paper, and hope that if this moets his eye he will favonr me with a few remarks on the subject. For many years various persons have attempted to ntilize electro-magnetism as a motive force with various success, never arriving, however, at what might bo called a useful machine. Now, from what I can see, the difficulty appears to lie in the fact that the influence of the magnet is only exerted within a very short space, and hence most machines have been intermittent iu their action,—a serious drawback. I think, however, I have overcome this difficulty completely, but am astonished to find that I do not get the

results which acknowledged laws of electro-magnetism would have led me to expect.

I will state a few of the idea« which crossed my mind, and inclined me to believe that, notwithstanding the repeated failures, such a machine was capable of being constructed so as to give a useful effect for the consumption of fuel. My first observation was that all motive force was the direct result of chemical action, and that for a definite amount of action a definite amount of motive force was produced, though not always in such a way that man could seize on it for practical purposes. Now the electro-magnetic machine being uppermost in my mind, I asked myself. Is not a steam-engine an application of electricity? My нъ - и i was, certainly. A coal fire is nothing more nor lit-;a than an electrical battery where the electricities the moment they are set free are reunited, causing heat and nothing more. Now this heat is the total result of the battery, and if we could ntilize the whole of this heat we should be utilizing the whole useful effect of в. battery consuming so many atoms of matter represented by the weight of coal and oxygen from the air. But every engineer knows how small an amonnt of that useful power is obtainable. If we look now at the other form of battery with acids and metals, where we can apparently collect the whole effect of consuming, or chemical action, in two wires, we naturally suppose that if we had a machine which would use up all that we conld give it from these wires and produce a nseful effect, we should have a machine which would actuallj cost less than a steam-engine, or rather would gave ж greater power in proportion to the weight of fuel consumed. Now I imagined, when I had overcome the difficulty I mentioned before, I had a machine which would not be far from giving the desired result«, and that by converting the high speed which ought to be given to a slower, but still nseful speed, I should do well. I have seen small wheels revolving several thousand times in a minute, but when I come to a drum whose diameter is loin., and whose weight with flvwheel is 1001b., although it has a perfectly continuous power of at least 301b. acting on its circumference, it will not give mors than 80 revolutions per minute with a battery of 40 cells (sulphuric acid only). Why is this? I cannot understand it.

I might state that by my arrangement the current has to make 1,000 trips on four separate wires alternately when the drum makes 8U revolutions. The irons of the magnet poles are Sin. diameter.

Тяга KEP..


[277] Sir,—If you would grant me space, I think it a duty to warn any querist very earnestly against the quackery rife on tliis subject, and which leads honest, but inadequately informed, writers to make such replies as the two in your current number (p. 501); the first wholly erroneous and mischievous in every point from beginning to end, and the second misleading more than it informs. I would say, let no one erect a conductor by advice of others, or till he has at least mastered as much'of this very easy subject as he may injan hour or two from the shilling •' Electricity" in Weale's series, or some other work of the late Sir W. S. Harris, by whose perseverance one class of works, but one only, the ships of the navy, were made lightning-proof. Hardly a building, small or great, ia ever now erected that might not be just as absolutely protected, and this generally at half or a quarter the expense that is incurred—when any is—for partial or nominal protection. And I do not know a habitable or used structure in England thus really protecteíl, or at present likely to be so.

Taking the two replies on p. 501 as texts, the points most necessary to be contradicted are these:—1. For "Q. Yorke's" italicized word " sufficient turf act" read sufficient?'« to or capacity; and understand that a certain minimum area of section, varying with the nature of the metal, is necessary. That is to say, for safety every part of the conductor must be of greater area than the thickest piece of that япте metal ever known to have been melted by lightning in this climate. This is not the case with F. Bedford's "j}in. iron wire," which would therefore be *imply a »onrce of danger, and far worse than no conductor at all. That size is aboat tho minimum that conld be held safe in pure copper, which is six times more conductive than the best iron and twice as much so as commercial copper. The real minimum of iron that should be allowed is a full square inch, or of copper a third as much, or*aya quarter, if of very good quality. It is impossible, therefore, yon see, for м/с copper conductors to be supplied, as "Q. Yorko" fancies, for Is. a foot, though iron ones might be. And whatever might be the case, could wo get copper made for the purpose chemically pure, as this is not attainable, copper, such as we can get now, is not so cheap, or so effective for a given cost, as iron. Zinc has a greater conductivity than iron, but yet would not be safe in smaller quantity, because we must insure it against heating to its melting point, which is not a quarter so high as that of wrought-iron.

2. Remember that no pari of the conductor mnst be thinned or narrowed to less than the safe capacity. Where it is joined the pieces must bo in flat contact over at least a square inch. If rounded, or touching only like the links of я chain, tho flrxt serions flash passing will fuse and destroy the contact, and the second will have to leap the interval by a fiery explosion. Mr. Walker's tapering steel "attraetor" again will, the first time it is struck, be scattered in molten drops, to tho imminent danger of whatever they touch.

3. As for an "attractor," it has been utterly disproved, these fifty years, that points can attract lightning, were this desirable. They attract the harm

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