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the penumbra was unusually pale, and the umbra 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 apertare. A very remarkable circumstance in connection with the sun's spots during the last two mouths has been their extremely light colour. The light brown tints of the nmbrii have been very marked, and totally different from the dark hues they usually present, while at times the penumbra; have been so light as to be scarcely visible. In most of theiu, 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 to.be 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 the 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. Lockyer) to the thinness of the solar envelope at the present time. This would certainly account for the light hues of the umbra; and penunibrie, and Oibo for the frequency and blackness of the nucleus. Mr. Henry Ormestier, Manchester, writes:—" On the 31st July, from 2h. 15m. to 3h. Om., 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 hide. I therefore determined to examine it with my 5 J in. refractor. I did so, using a power of liso. 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 examined the cluster generally and was struck 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 the upper part, appeared brighter than others, presenting a very mottled appearance." Mr. William F. Denning, of Bristol, observed the sun witlrhis Sin. refractor, July 14th, from 5h. 80m.to6h. 10m.; he noticednine] well-defined macula on various parts of the disc. A particularly large and interesting group of spots was visible in |the N. hemisphere. On July 22nd, at 8 p.m., a spot was observed in the 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 bridges seemed to present the appearance of running matter. This observation was made with power 100 on a 10$ih. reflector, by Browning.

The Lunar 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, hut 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 Bhadows 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 Hky was clear at first, with u small amount of stratus near the horizon. I first caught sight of the moon at 8.41, but it waB 8.49 before it got clear of the clouds. I paid particular attention to the degree of distinction with which the eclipse portion could be seen. When about fon r digits were covered, I just noticed the copper tint through the telescope. I fancy this would be a little sooner than in the last 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 1.85, when I noticed the marc tranquillitatti and the mare BcrenitatU showing with beautiful distinctness through the earth's shadow in the telescope. A few minutes after the totality was attained, I was struck with the obscurity of the eastern side of the moon being so much more than I had expected. At 9.55 at least half of its surface was as if blotted out, even when seen through the telescope, although I applied two different powersID' and 150Q. Three of the teas at the western side were all I could make out. Possibly a thin coating of cirrus cloud which covered 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 seen 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 Way. At 12.24 the lunar circle was complete." Mr. Oliver J. Lodge, of Hanley, reports that " the colour of the moon during the totality was of a most peculiar copper hue, giving very little light indeed. But during the egress of the shadow it was almost as white and silvery as it usually is, although still under the penumbra." Mr. Edmund Neison, of London, says:—" The colour of the eclipsed disc was during tho 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 tho fog banks, AritUirehtt* was quite distinct as a bright crater, and even before eleven GrimaUii 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 wire distinctly visible. Tho copper tint was also very evident. During a portion of the time the moon wa*> overcast with clouds.

Venus.—Mr. Henry Ormestier, of Manchester, observed this planet with f»Jin. equatorial refractor on July 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."

Saturn.—Mr. H. Michell Whitley, of Penarth, Truro, writes:—" July 7th, lOh. 11m., power 206°. The ball of tho planet dull yellow colon r. N. equatorial ruddy belt conspicuous, and another of same colour between it and pole; pole bluish grey. Edges of disc fainter than centre. Hky in Ansa-much blacker than around planet. Crape ring across bull nearly as dark as hall's division, palo purple. Crape ring Very easy in Ansa*. No line of light between it and B."

Occultation.—Mr. Joseph C. Lambert, of Sleaford, witnessed the occultation of B.A.C. 5y54, on July in, and found the exact time of disappearance to be l'Jh. 40m. 41s. mean time.

Meteors.—Mr. J. C. Lambert "observed a very brilliant meteor at llh. 40m., July 21. Course from a little below 7 Cassiopeia; to f IVrsei. 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 « Bootis to 43 Conine Berenices, and immediately afterward one from a little below 43 Coiua Berenices to n Bootis. Could this have been one and the same meteor describing an arc?"

Lunar 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.

LETTERS TO THE EDITOR.

CWe do not hold ourselves responsible for the opinions of our correspondents. The E Ditor respectfully requests that all communications should be drawn up as briefly as possible.]

*»* All communications should bo addressed to the Editor of the English Mechanic, 31, Tavistockstreet, Covent Garden, W.C.

All Cheques and Post Office Orders to be made payable to J. Passmore Edwards.

"I would have every one wrlto what he knows, and as much as he knows, but no more; and that not in this only, but in all other subjects: For such a person may have some particular knowledge and experience of the nature of Buch a person or such a fountain, that As to other things, knows no more than what every bod v does, 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's £uay*.

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

THE SATELLITES OF URANUS.

[257] Sir,—Sir John Herscbel gives elements according with Sir William's account. Will" Etudiant" quote Mr. LasselTs exact words? I cannot but think there is some misapprehension. In 1*51-2 the deter* mination of the place at which a Uranian satellite reached its greatest elongation would be a matter of extreme difficulty, since the apparent path of the satellite, so long after the Urauiun equinoxes, would be an ellipse of but moderate eccentricity. The difference of apparent distance when the position-angle of Uranus was 187J0, and 172$ would not bo great.

Kichd. A. Proctor.

CURIOUS QUESTION.

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Sir, — The circumstance referred to by (see 235 and 28b") 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 resistance 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 due cast, and a perpendicular from each extending throughout the motion, to the ground; then the motion of A's perpeudicular along the ground will exceed the motion of B's, while the balls are rising. And, therefore, if A and B could be suddenly stopped at the 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 tho descent those conditions are exactly reversed, and at the moment of reaching the ground A would be as far to tho west of the point of projection as B to the east. But if we take into account the resistance of the air, wo have

a more complicated matter to deal wiih. I ^ time to go very carefully into the profclgR, rj probably one of some difficulty; but I U&^-|._ found that the westerly ball will bars ^/~ greatest range,—for this reason, that thpv,' motion is greater while each projectile b x:i while it is falling, and the gain of the baUAn^* is therefore greater than the gain of tV u. * falling. But though here the chief tH-c «, has been considered, ».<., the action ot It 4-^_ ance in modifying the shape of the tra-.-, effect due to another cause has not Wt: account,—I mean the fact that the ov^*,, air are carried somewhat more swiftlj tfe^ being at a greater distance from th* -.^ Owiag to this, the ball (B) which is fir*-* (> east encounters the horizontal re*ist*i_- . horizontal velocity, minus the tliffertiicr -^ motion of a point at the earth's sorfae*, I?j point considerably above tho earth's ec&. equator); while the ball (A) fired west ward & horizontal resistance due to the sum of thr-v Combining this with the accepted rtkL.r velocity and resistance, wo can form an *. the effect of this cause in diminishing the m ball A as compared with that of the fasl. balance must be struck between this effs^. former. But the only way of ready decidiM -. tion would be to take the equations of mot^L least partially integrate them. I have the or. before me, but I do not like their look braarsr.

RlCHD.A. PluT

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ON THE LINK MOTION.—Letter II.

[•261] Sik,—Engineers generally try to get in long exeentric rods, and in ordinary locomotive practice the length of the rod is often as ranch as twelve times the throw of the exeentric, and seldom I believe less than ten times; bat in marino screw engines and in some paddle-wheel engines they are compelled through want of room to use very short rods, and lengths of five or six times the throw are common.

In designing a link motion the first things to be considered are the proportions of the slide-valve, and they are of coarse determined by the capacity of the cylinder and the speed of the piston. Suppose it is determined for a given engino to have a valve with Jin. of lap, and 4$in. of travel, and Jin. of lead in full throw, the next things to be determined are the lengths of the exeentric rods and the length of the link. Say that space limits as to a 3ft. rod, and as thero is no law for the length of the link any more than the exeentric rod, we will set it down at J.2in. from centre to centre of exeentric rod pins. The " lines " of this motion should then be laid down on the drawing board in the two positions shown in Fig. 8. Draw tho main centre line C V as that which passes through the centres of tho crank shaft and valve-spindle; let C be tho centre of the crank shaft, and round it draw a circle 4 Jin. 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 the line C V, as the temporary positions of the centres of the forward and backward excentrics; draw two lines H and K 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 3ft.— the length of the exeentric rod—in the trammel; Bet one leg in centre F, and mark with 1 the point of intersection of the other leg with lino H, and with 4 its intersection with line K. Then set the trammel in centre H, 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 centreline C V, and draw the arc 1, 2; do the same with point of intersection 8, and draw arc 8, 4. These two arcs are the trne curvation of the link, and their distance asunder is the amount of motion caused by the crossing of the rods : in the present case it is Jin., •r Jin. on each side of the point M D, which is the

•mean distance 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 stroke, covering the ports seven-eighths of an inch on each end. For distinction sake let us call these two positions of the link the petition* of no lineal 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 C and parallel to F B, and where L A intersects the circle atF' and B' there are the proper positions for the centres of the excentrics. Ouce more take the trammel—still set at 8ft—and place one leg in F', and mark point 5 where the other leg cuts line H ; do the same with point B', and mark point 6 on line K ; draw the arc 5, G as before directed, and there we have the position of the link when the crank is on the back centre at C. 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 be seen that there is a distance of 2 jin. 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 2fin., or lj-Jin. on each aide of the "mean distance point." But the valve covers the port only Jin., and therefore, when it is moved lgin. 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 is in "middle gear," and only £in. when in "full throw " with the end of the link opposite the valve-spindle.

In this position, Fig. 4, the forward gear exeentric rod is exactly opposite the valve spindle, which therefore derives its motion exclusively from this rod for the time being. The apparatus now assumes tho simplo character of a valve driven by a cowman tingle exeentric 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 tho " positions of no lineal advance" 1, 2, 8, 4, which are coincidental the forward gear end, Jin. asunder at the middle, and ljin. asnnder at the back gear end of the link. But that does not increase the total motion at that end, which is ever an<* alwav9 oqnal to the throw of the exeentric, notwithstanding what Mr. Harrison says

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

Hitherto I have spoken only of that arrangement of link motion in which the lead is greater in "middle gear" than in "full throw." I shall now glance at that arrangement which produces a contrary effect on the lead, making it leu in "middle gear." This is illustrated in Fig. 5. In this figure all dimensions are the same as in Figs. 3 and 4, and the same letters of reference are common to both, the only difference being in the disposition of tho rods, which is e< utrary to that of 8 and 4, for in this the back gear rod is attached to the upper end of the link and the fore gear rod to tho lower end. The vibration at the ends of tho link is still of course equal to the throw of the exeentric, and the amount of motion transmitted to the valve in full throw the same as in Figs. 3 and 4 ; but the motion at the centre of the link is considerably reduced, for it is now only lgiu. or 11-lCths on each side of tho "mean distance point;" but the valve has jjin. of lap, and would require to move that much on each side of its middle position before it would begin to nncover 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," it is actually &-\6ths blindat the centre, or in " middle gear," and in order to get Jth of lead in that position, we must cause the centre of the link to vibrate 2in. (think of that Mr. Harrison), and to do that the "lineal advance" of the excentrics must be increased from lin. as at present to about 1 G-16iu., shown in dot lines, and then the lead of the valve when in full throw will bo about 7-10in.

Now what is the reason of all this? Why is it that in Figs. 8 and 4 the vibration of the link at the centre is so much greater than it is in Fig. 6, the lineal advance and every other dimension remaining equal? The crossing of the rods produces a similar effect in both cases, viz., Jin., as is shown by tho "positions of no lineal advance " 1,2,8, 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. 8, extracted and laid down separately for sake of greater distinctness. C and C" show the crank on the two dead centres; F' and F are the corresponding positions of the centre of the forward gear exeentric, the rod of which is carried to the upper end of the link; and between these two positions of the end of the exeentric rod there is a space of 2 jin.

Now look at Fig. 7. It is a portion of Fig. 5, showing the forward gear rod attached to the loiter end of the link. In this case there is a space of only lijin- between the two positions of the end of tho rod, and as tho 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 tho valve is a grave defect of the link motion,—the shifting link motion, I mean, such as we have been considering hitherto; but the stationary link is free from that defect. The "shifting link " is so called because 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 nam© implies, is a fixture, and is suspended or sustained from some fixed point, and has no motion except that imparted by the excentrics. The reversing lever has no connection with this link, but tho engine is reversed or expansion altered by means of a rod called the "valve spindle connecting rod " one end of which is jointed to tho head of the valve spindle, and the other end carries the slide block, winch rite in the slot of the link. The reversing lever is connected with this "valve spindle connecting rod " by any suitable means, so that the attendant can place tho 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 the reverse way to the " shifting link," for it is set with it* concave side towards the valve spindle, and the radius of its curvature is the length of the "valve spindle connecting rod," but 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 th« 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 the construction and working of the "link motion," in place of the erroneous ideas, the absurd crotchets, and tho fantastic notions with which his head was so full when he wrote his notable letters in No. 272, page 352, and in No. 280, page 471.

James Baskebville.

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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.

IRISH MOSS ANT) COD-LIVER OIL.

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.

OUR SIZE AND FORM.

|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.

TELESCOPIC.

[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.

READINGS FROM THE GLOBES.—X.

[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 iary, and during that interval of two mouths the a. must necessarily bo above the horizon some part Lo time, and if it affords no warmth it will at least r*l li^Iit. The intermediate strips or belts, how, will uovor bo totally deprived of twilight, bokc only tiioso places within 5£" of tho polo will bo Ll\i£ titato. Again, in the temperate zones the udtikry line whero places can have no twilight all it la 4H£ uorth and south latitudes, and this is ^irt«hino<l iu tliis way :—As tho greatest declination 111'\ if we add to this 18 , tho extent of twilight, we IX liavo 4113 as the distance from the pole of all cea vrhich enjoy this boon in the summer time; conixerttly, by aabtracting this from i>0 , we shall have , J &a the parallel where twilight all night ceases— ut is, only tuone places whoso latitudes exceed Quebec t^Lxu western hemisphere, or Paris and Vienna in the storu, can have twilight all night. With respect to 3 south polar regions, the poets of antiquity (see jveorfricB," Book I. lino 211,-v.. i could offer coujecrea only, assigning to them the abodes of tho inrnals and the presence of perpetual night; but odern science can offer reason* why the south polar gions should be enlightened during their night in the jue maauer as their antipodes, and regards such irttirtious &a arising from the defective knowledge of ie ancients on those points, aud as embellishments for kxeir tine poetry.

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T. S. H.

HEIGHT OF CLOUDS.

t'207j Sib,—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 Angnst, is deficient of date, so that the most important clement of an explanation, viz., the age and position of the moon, is wanting. The moon might have been Bo near tho horizon as to illuminate vapour in the higher region of 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., 1W6D, in latitude 55~ 46' North, longitude -iJ 0' West, the moon in the 21st day of her age, and declination 20 58' North, being E.N.E., near tho 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 shoue tolerably bright, except here and there near the horizon, which was irregular in its outline, partly from clouds aud 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, mot 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 1 cannot say, but it continued fully half an hour quite distinct, and gradually faded away. I concluded then, and 1 am of the same opinion still, that it was the moon's light, reflected from vapour in the upper regions, so disposed in the atmosphere as to present to my eye the form of a somicircalar bow of light in a sky only faintly illuminated by the moon.

J. Steel.

HORSE POWER.

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

Horse Power.—1. Given a vertical semi-portable fire-tube boiler, with internal fire-box and vertical fire* tubes, 4ft. high, 1ft. Bin. in diameter ; firebox 1ft. 5in. high (between furnace bars and crown of fire-box), by 1ft. 3iu. diainotur, 7 fire tubes 1ft. llin. long by 2Jin. outside diameter; water in boiler lOin. deep over crown of tire-box aud 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. Coals of good quality. Required:—

(a) Quantity of steam of 601bs. pressure (75 from a vacuum J obtainable from this boiler per hour i

(b) If the tube surface above water line is as effective as that below in the way of producing steam? and,

(ci If a greater quantity of steam would be made and how much more by raising the water line to 15iu. above crown of fire-box?

2. Rciuured olrio to know, if for this quantity of steam (sub. 1 a) a non-condensing engine of 3jfin. diameter of cvliuder and 7in. stroke, with common slide valve cutting oil at (say) £th stroke, is suitable; and what would be tho proper velocity of ptiton, or number of single strokes per minute, in order to correspond with steam supply from boiler (as above); and what would be the theoretic, ami what the real or effective horsepower thus 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 aud highly-instructive journal, aud hope, therefore, they will be forthcoming from those of your correspondents so well able to deal with instigations of this kind.

A. \V. E.

ISOMETRICAL. [269] Sin,—I wish to draw the attention of "Isometros"to the circumstance that your engraver 1ms reduced my drawing (p. 524) to mako it tit your columns, aud cnriously enough just sufficiently to make the line A O in Fig. 2 measure lin., whereas'the projection of that line—viz., A O in Fig. 1—is described in the text as lin. long, which, without this explanation, would make it appear that I had overlooked his requirements under the N. B. in his query on p. 478.

J. K. P.

FIELD BOILER AND PRIMTNG[270] Sir,—Will some of my brother readers kindly advise me in the following difficulty?—Amongst several of tho 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, lift. 2in. high by Oft. 7in. outside diameter; firebox (with water space all around of 6in.), 6ft. 2in. high by 5ft. 7iu. diameter; 4i> Field tubes 8ft. 7in. long, and W7 do. ;(ft. lljiu. long, 2Jin. outside diameter; total heating surface, 691J square feet; nominal power of boiler, 50 horses; working steam pressure, 751b. per square inch; depth of water over tire-box crown. lHiu.; steam space, 3ft. 6in. high; smoke uptake pipe passing vertically up through steam space; draught very brisk; steam to engine taken off close to top of boiler by half-circlo 4in. pipe, having long slots on upper half of surface about £in. broad by 2in. long each; feed enters cold near bottom of boiler, and is seemingly pure and quite clear. Now, although other boilers of like construction and

{jroportions work satisfactorily, this one prime* so very »adiy that, with otherwiso efficient feed pump full on, it is impossible to keep the proper level of water in the boiler, besides which the 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 cause—taking into consideration that in other similar cases it is not found to occur. Can it bo 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 ou 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 theoretic H.P. (t.*., horse power); / the area of piston in square inches. Then, again, further on, R is used instead Of H.P. (horso power). In query 3, H.P. (horse power) is also to bo understood by It, wrongly printed, and tho words within tho flrBt brackets of this query should be (high and low pressure). Then, again, tho words " OB indicated by an open mercurial " wants the tho concluding word "gauge" and the right-hand bracket. In tho place of "cylinder respectively," it should be read " cylinders respectively."

THE NATURE AND CAUSE OF LIGHT.

[271] Sir,—Being profoundly ignorant of the nature of light—vide Mr. Proctor's letter, p. 317—perhaps he will kindly explain the cause of two curious photographic phenomena. If I take a sensitised collodion plate, and cover it with orange-yellow glass, exposure to the 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 tho first.

As Mr. Proctor thoroughly understands this subject, I shall osteem it a favour if he will enlightcu my ignorance of tho 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 aro, however, worthy of no consideration. T. A.

EMIGRATION.

[272] Sir,—Obviously the aim of all thoughtful men at the present moment should be to keep the subject of emigration as much as possible in tho background. The disturbances on the Continent have created work for the unemployed. Our arsenals and dockyards again team with lifo and activity; the harvest requires labourers; the drain of German and French subjects from our shores causes vacancies in our warehouses which must, if not permanently, at least temporarily bo filled up. Being impressed with these ideas, my intention was to rafrain, for the 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 tho 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 us practical farmers, or," &c., and " this would iu no way exclude such mechanics as smiths," 4c. If tho advice given were followed, viz., that a/annfTi smith, and carpenter should emigrate hi batches, T think emigration would soon bo stopped

from the want of practical farmers. Airain, as a rule, it is not tho last-mentioned class who emigrate, or who benefit to any great extent by emigrating. Further, pausing at his declamation against Canadian seasons and hardships, is it not a fact that the American winter and lumnur are also extremes? Is Western America a panacea for all the evils uude.r the sun? The prairie, I suppose, offers no hardships, it is well intersected by good roads, comfortable houses may there be found in medium quantities, -irater may at all times bo obtained, the lied 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 n, poor mechanic to do in order to gain comfort and competency is to accept advice, gratuitously and senteutiously 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 sentences 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; ho 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. G.'' certainly is extremely vague in the following sentence: —" He can see the lands himself and make hit bargain, then, if he is satisfied, let him send passage-tickets to his friends." Would it not be best to make the bargain after being satisfied with the land, ice, 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 " crops up again, and tho result is worthless advice. The emigrant's policy must be to leave these shores, so that ho 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 tho American prairie, giving its drawbacks as successfully as he does those of Canada, his letter would have been prouonnced excellent; but holding the view as I do rogarding 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 better information than that contained in the Government pamphlet, entitled "Information for Emigrants to the British Colonies," 2d.

F.R.G.S.

[" 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.]

BICYCLE RIDING.

[273] Sir,—I have read, with some surprise and considerable amusement, a letter from Mr. R. G. Bennett, in your issue of the 12th inst., in which he asserts the bicycle to be a mere toy, and useless for all practical purposes. This opinion ho 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 sufficient to thoroughly exhaust himself on a twenty- mile 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 intimidateothers from learning the machine.

The bicycle club to which I belong always travels at the average of eight miles an hour for the whole journey. When the rood is down hill, we frequently run, for some two miles, at the speed of twelve to fifteen miles per hour ; uphill, we work at from four to six miles an hour; ou level roads at seven to eight,. or even nine miles an hour if the ground is smooth. If I go out with only a 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 tiino since, by on "elaborate calculation," that the force necessary for walking a distance of five miles, if put into use on the machine, would only carry three miios, or little over two-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 so unscrupulous an arithmetician. But I should be most happy to afford Mr. Bennett the opportunity of praeticaUy testing the "elaborate calculation" before mentioned, by matching my bicycle against his logs for a day's journey. I may mention for his comfort, that I could travel sixty nines in a 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 tho "elaborate calculation "J, it follows, of course, that to prove this theory he would require to walk one hundifl mile* a dap. I hope bo can. I may add that I am by no 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 twolve months' practice.

I cannot, of course, account for Mr. Bennett's illMicv- 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 hare been learning for yean, and go into the water a hundred times in a reason, while others acquire the art almost instinctively. But this is no reason why I should set 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 quite 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 duluess. I would have Mr. R. G. Bennett take to himself the same cosnfort. W. E. Mavkrly.

[274] Sir,—Your correspondent B. 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 abunrd statement. I can, however, bring the testimony of a year and a half's bicycle riding to bear out 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 subject 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 circumstances, buch for instance as the crossing of rough country, is to use the legs with which nature has provided us, supposing of course tiiat the motive power is supplied by the traveller 1 im elf. Our natural powers of locomotion are intended to be, as they in renlitv a~e, perfect in their adaptability to diversified circumstances. It is nnder partia I ir circumstances that art steps in and for the tune being, supersedes nature. Thns 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 cose, or, in other words, is the quality of our roads such as to give bicycle riding the advantage ovur walking to the majority of people? 1 maintain that the trne 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 Mill go tearing about to the risk of their necks ; neither ought we to accept the testimony of those who naturally speak in favour of the machine, because they bappen to live where the roads are as smooth as a board. The real answer to the bicycle question can only be given by the general public, and their opinion can only be measured by the use which we see can be made of the machine. It is now about a year since I wrote au«l predicted the decline of the bicycle fever, and I think we need only look about us 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 fact 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 could wish. It was to be ridden at the rate of 8, 10, or 19 miles an hour, and was, in fact, to go almost "by itself." The experience of a couple of years has, I think, dispelled the illusion, and the instances in which the bicycle is put to any really practical use, such for instance, as a business man travelling to and from his office, or a 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 in 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 ease and comfort, getting any practical nse out of it on our roads, just as they happen to come, I am afraid he will he doomed to disappointment, and will be like many other-, 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 back 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 I 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 quite agree with Mr. B. that some of yonr 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 np that speed for any distance, say for ten or twenty miles; or oulv that they can for a short distance ride at the rate of ten miles an hour? I fancy the latter is the real state of the case. Very few riders can, I am sure, keep np a higher rate of speed than six or seven miles an hour

f< r any distance. I hare frequently travelled eleven meuured mil*--, containing a fair amount of np and down hill, in an hoar 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 fire miles an hoar. I am quite aware that Messrs. Jones, Brown, and Robinson have occasionally gone 100 miles in a day, and have, moreover, appeared at their de->ks the next morning as if nothing had happened—their spirits, no doubt, being kept op by the pleasing thoughts of seeing themselves immortalized in the papers; bat performances of this kind may very well be classed with such feats at walking 1,000 miles in 1.000 hoars. 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 amount of vital energy, which would require a great deal of rest and food for it- 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 jnst in the same proportion as one or more cf these favourable conditions fail, in just the same proportion does bicycle riding become a " toil of a pleasure ;" and it is the improbability—I may almost say the impossibility—of making these favourable conditions universal, that renders impossible the universal adoption of the bicy cle. W.

MORDAK'S SELF-CENTERING WIRE CHUCK.

f'275] Sir,—As requested I herewith send 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 month description, never having Kp 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 C to guide the cap and keep it trne, it 13 a great improvement.

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ELECTRO-MAGNETISM.

f'2761 Sir,—I have read with great pleasure some articles by " Sigma" in your paper, and hope that if this meets his eye he will favour me with a few remarks on the subject. For many years various persons have attempted to utilize electro-magnetism as a motive force with various success, never arriving, however, at what might be 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 in their action,—a serious drawback. I think, however, I have overcome this difficulty completely, but am astonished to find that I do not get th0

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LIGHTNING CONDCCTOES. £277] Sir,—If yon would grant me space, 1 T£ii it a duty to warn any querist very earnestly isc4 (lie quackery rife on this 6abject, and wukfcjsfc honset, but inadequately informed, writers ('-*'■ -uch replies as thetwo in your current number if. Kir. the first wholly erroneous and misehiennis is rer> point from beginning to end, and the tecosd Sj> leading more than it informs. I would say. ht M «* • rect a conductor by advice of others, osttUh* &** -' loast mastered as much'of this very es/y wabjett A* be may injan hour or two from the «biifinf "Cecfriarr" in Weale's series, or some other war* of uV late oat

> W. S. Harris, by whose perseverance one class ol works, but one only, the ships of the navy, were made lightning-proof. Hardly a buildup small Ot grtai, i» ever now erected that might not be \os>\ a* aawtesdj

t protected, and this generally at halt or a Quarter the

i expense that is incurred—when any U—tor partial or nominal protection. And I do not know a habitable « used structure in England thus really protected oi **

1 present likely to be so.

Taking the two replies on p. ,101 a* texts, the poini* most necessary to be contradicted are these:—1. ** "Q. Yorke's" italicized word "sufficient eurfocf,'' rf*i -nrficient bulk or capacity; and understand that acerac minimum area of section, varying with the nature^ t lie metal, is necessary. That is to sav. for stfrP every part of the conductor mast be of "greater *■•* than the thickest piece of that name metal ever %&*

I to have been melted by lightning in this climate. T* is not the case with F. Hertford's "?in. iron ss»' which would therefore be simply a tourer o/'da****.1* far worse than no conductor at all. That aiae is sV*1 the minimum that could be held safe in purr (**>' which iB six times more conductive than the besr* and twice as much so as commercial copper. S real minimum of iron that should be allowed is *~ square inch, or of copper a third u- nui.-h. or*!1 <inarter, if of very good quality. It is impost therefore, you see, for $afe copper conductors tt- si supplied, as "Q. Yorko" fancies, for Is. a foot, sastf* iron ones might be. And whatever might be the os* could we get copper made for the purpose iUUa»alj pttrv, as this is not attainable, copper, such aswr**2 get now, is not so cheap, or so effective for a given co* as iron. Zinc has a greater conductivity than iroebut yet would not be safe in smaller quantity, becso* we must insure it against heating to its melting point which is not a quarter so high as that of wronght-ir?!!

2. Remember that no part of the conductor must b* thinned or Jiarrowed to les9 than the safe capacity Where it is joined the pieces must be in flat eontad over at least a square inch. If rounded, or touching only like the links of a chain, the Jlr»t serious flash passing will fuse and destroy the "contact, and the second will have to leap the interval by a fiery cv plosion. Mr. Walker's tapering steel" " attracW" again will, the first time it is struck, bo scattered ia molten drops, to the 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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