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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 aperture. A very remarkable circumstance in connection with the sun's spots during the last two months has been their extremely 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 penumbræ 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 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 umbræ and penumbræ, and also 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 3in. 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 54in. 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 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 with his 3in. refractor, July 14th, from 5h. 80m. to 6h. 10m.; he noticed nine/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 in. reflector, by Browning.

was fairly above the fog banks, Aristarchus was quite
distinct as a bright crater, and even before eleven
Grimaldi 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 were distinctly
visible. The copper tint was also very evident. During
a portion of the time the moon was overcast with
clouds.
VENUS.-Mr. Henry Ormestier, of Manchester, ob-
served this planet with 54in. 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, 10h. 11m., power 208.
The ball of the planet dull yellow colour. N. equa-
torial ruddy belt conspicuous, and another of same
colour between it and pole; pole bluish grey. Edges
of disc fainter than centre. Sky in Ans much blacker
than around planet. Crape ring across ball nearly as
dark as ball's division, pale purple. Crape ring very
easy in Ansæ. No line of light between it and B."
OCCULTATION.-Mr. Joseph C. Lambert, of Sleaford,
witnessed the occultation of B.A.C. 5954, on July 10,
and found the exact time of disappearance to be 12h.
40m. 41s. mean time.

METEORS.-Mr. J. C. Lambert "observed a very
brilliant meteor at 11h. 40m., July 21. Course from a
little belowy Cassiopeia to Persei. Nucleus appeared
as a star of 15 mag., tail nearly 2 long.; colour
yellowish white. Duration 2 seconds. During the time
of observation, 11h. to 12h. 30m, I observed no less
than eleven small meteorites. The course of one of these
was from Bootis to 43 Coma Berenices, and imme-
diately afterward one from a little below 43 Coma
Berenices to 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.

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

All communications should be addressed to the EDITOR of the ENGLISH MECHANIC, 31, Tavistockstreet, Covent Garden, W.C.

All Cheques and Post Office Orders to be made pay. able to J. PASSMORE EDWARDS.

"I would have every one write what he knows, and this only, but in all other subjects: For such a person as much as he knows, but no more; and that not in may have some particular knowledge and experience of the nature of such a person or such a fountain, that as to other things, knows no more than what everybody 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 Essays.

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

THE SATELLITES OF URANUS.
[257] SIR,-Sir John Herschel gives elements ac-
cording with Sir William's account. Will Etudiant"
quote Mr. Lassell's exact words? I cannot but think
there is some misapprehension. In 1851-2 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

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, 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 distinction with which the eclipse portion could be seen. 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 last eclipse I observed (September, 1867), 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 tranquillitatis and the mare serenitatis 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 powers-satellite, so long after the Uranian equinoxes, would be 70° and 150°. Three of the seas 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 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

an ellipse of but moderate eccentricity. The difference
of apparent distance when the position-angle of Uranus
was 1871, and 1724 would not be great.

RICHD. A. PROCTOR.

CURIOUS QUESTION.

[258] SIR, The circumstance referred to by
"M. L.," (see 235 and 236) 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
ance 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 perpendicular 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 the descent these con-
ditions are exactly reversed, and at the moment of
reaching the ground A would be as far to the west
of
the point of projection as B to the east.
take into account the resistance of the air, we have
But if we

a more complicated matter to deal with. I
time to go very carefully into the proble
probably one of some difficulty; but I belieR.
found that the westerly ball will have
greatest range,-for this reason, that the br
motion is greater while each projectile i
while it is falling, and the gain of the ball As
is therefore greater than the gain of the t
falling. But thongh here the chief edec
has been considered, i.e., the action of th
ance in modifying the shape of the tra
effect due to another cause has not bee
account,-I mean the fact that the u
air are carried somewhat more swiftly L
being at a greater distance from the
Owing to this, the ball (B) which is fire
east encounters the horizontal resister
horizontal velocity, minus the difference
motion of a point at the earth's surface, 1
point considerably above the earth's
equator); while the ball (A) fired westwarde
horizontal resistance due to the sum of ther
Combining this with the accepted relat
velocity and resistance, we can form an
the effect of this cause in diminishing the re
ball A as compared with that of the ball!
balance must be struck between this effects
former. But the only way of realy deciding i
tion would be to take the equations of motion.
least partially integrate them. I have the o
before me, but I do not like their look by any m
RICHD. A. PE

2

THE IRIS DIAPHRAGM. [259] SIR,-In reply to "Unit" query 445 forward a sketch of the only plan known to me, ha never seen any other. Of course the details of rc. struction may be varied indefinitely to sit difer purposes and positions. It consists of five thin metal leaves pinned on to a back plate, on which a louse rar slides by means of slots and studs, carrying aim the studs that take into the slots in the tails of the five leaves. Washers must be provided under the piss of the leaves to make up the thickness of the The entire thickness will be about 3-16 essential that the broad end of the lear order to close well.

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ON THE LINK MOTION.-LETTER II.

[261] SIR,-Engineers generally try to get in long excentric rods, and in ordinary locomotive practice the length of the rod is often as much as twelve times the throw of the excentric, and seldom I believe less than ten times; but in marine screw engines and in some of room to use very short rods, and lengths of five or six paddle-wheel engines they are compelled through want

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 course determined by the capacity of the cylinder and the speed of the piston. Suppose it is determined for a given engine to have a valve with in. of lap, and 4 in. of travel, and fin. of lead in full throw, the next things to be determined are the lengths of the excentric rods and the length of the link. Say that space limits us to a 3ft. rod, and as there is no law for the length of the link any more than the excentric rod, we will set it down at 12in. from centre to centre of excentric rod

M+D

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mean distance of the centre of the link from the centre of the crank shaft, and the centre of the link will be at MD

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 positions of no lineal advance.

lel to F B, and where L A intersects the circle at F

5

The valve has seven-eighths of an inch of lap, and oneexcentrics. Draw line L A lin. from centre C and paraleighth of lead, giving lin. for the lineal advance of the and B' there are the proper positions for the centres of the excentrics. Once more take the trammel-still set at 3ft.-and place one leg in F', and mark point where the other leg cuts line H; do the same with point B', and mark point 6 on line K; draw the arc 5, 6 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 2in. 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 the in. of motion caused by the crossing of the rods, making altogether 23in., or 1ĝin. on each side of the "mean distance point." But the valve covers the port only in., and therefore, when it is moved 1ĝin. from that position by the link it must leave the port in. open, or in other words there is in. 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.

pins. The "lines" of this motion should then be laid down on the drawing board in the two positions shown in Fig. 3. Draw the main centre line C V as that which passes through the centres of the crank shaft and valve-spindle; let C be the centre of the crank shaft, and round it draw a circle 44in. 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.In this position, Fig. 4, the forward gear excentric the length of the excentric rod-in the trammel; set rod is exactly opposite the valve spindle, which thereone leg in centre F, and mark with 1 the point of inter- fore derives its motion exclusively from this rod for the section of the other leg with line H, and with 4 its in- time being. The apparatus now assumes the simple tersection with line K. Then set the trammel in character of a valve driven by a common single excentric centre B, and mark with 2 the intersection of the other and one rod. The effect of the crossing of the rods has leg with line K, and with 3 its intersection with line H. entirely disappeared from this end, but it remains unAgain set one leg of the trammel in point of intersec- altered at the centre of the link, and is doubled at the tion 1, and the other leg wherever it comes on main other end. This is plainly shown by the "positions of centre line C V, and draw the arc 1, 2; do the same no lineal advance" 1, 2, 3, 4, which are coincident at with point of intersection 8, and draw are 3, 4. These the forward gear end, fin. asunder at the middle, and two arcs are the true curvation of the link, and their 14in. asunder at the back gear end of the link. distance asunder is the amount of motion caused by that does not increase the total motion at that end, the crossing of the rods in the present case it is fin., which is : excentric, notwithstanding what Mr. Harrison says ever and always equal to the throw of the

or fin, on each side of the point M D, which is the

But

about" that end of the link having double the vibration," and the valve has the exact amount of lead (in.) due to its lap and the lineal advance; but from that position the lead gradually increases as we approach the centre, when the maximum of in. is attained, for there the whole effect of the crossing of the 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 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 less 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 the rods, which is contrary to that of 3 and 4, for in this the back gear rod is attached to the upper end of the link and the fore gear rod to the lower end. The vibration at the ends of the link is still of course equal to the throw of the excentric, 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 1gin. or 11-16ths on each side of the "mean distance point;" but the valve has in. of lap, and would 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 11-16ths on each side of the "mean distance point," therefore in this arrangement of the motion, when the valve has in of lead at the end of the link or in "full throw," it is actually 3-16ths blind at the centre, or in "middle gear," and in order to get th 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 5-16in., shown in dot lines, and then the lead of the valve when in full throw will be about 7-16in.

Now what is the reason of all this? Why is it that in Figs. 3 and 4 the vibration of the link at the centre is so much greater than it is in Fig. 5, the lineal advance and every other dimension remaining equal? The crossing of the rods produces a similar effect in both cases, viz., fin., 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. 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 excentric, the 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 in.

Now look at Fig. 7. It is a portion of Fig. 5, showing the forward gear rod attached to the lower end of the link. In this case there is a space of only 1gin. between the two positions of the end of the rod, and as 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 shifting link motion, I the stationary link is free from that defect. The "shiftmean, such as we have been considering hitherto; but ing link" is so called because in the act of reversing the the case may be; but the "stationary link," as its name engine or altering the grade of expansion, the link is moved up or down by the reversing lever or wheel, as implies, is a fixture, and is suspended or sustained from some fixed point, and has no motion except that im

parted by the excentrics. The reversing lever has or expansion altered by means of a rod called the no connection with this link, but the engine is reversed "valve spindle connecting rod" one end of which is jointed to the head of the valve spindle, and the other end carries the slide block, which fits 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 the slide block to work in any part of the link, either for reversing or expan

sive working.

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and scientific piece of work than the shifting link, and The stationary link motion is a much more elegant 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 the construction and working of the "link motion," in place of the erroneous ideas, the absurd crotchets, and the

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 BASKERVILLE.

GAS.

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On 1st August, 1870, the carburetter was emptied,
after being in constant use and untouched for sixteen

months; four pints of fluid remained, sp. gravity
785, which gave a power of light, with the same burner
and consumption per honr, equal to 59 candles. The
carburetter was then refilled, including the residuum
above mentioned, when the power of the 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.

[262] SIR,-I undoubtedly invited disenssion, not about gas companies, bat how to reduce our gas bills," and improve our light;" nor do I complain of fair and honest criticisin. Sigma" is far too able a chemist for me to attempt, if I wished, to argue the point he raises, even if you would allow your valuable space being so taken up. But I must totally differ as to his assertion that it is not even true that the illuminating power is reduced by the alleged re- The total of gas consumed in sixteen months was moval of illuminating hydrocarbons from the gas." 13,000ft. The number of burners in daily use was This is a bold assertion for him to make, and is also three, and occasionally six, with two small gas stoves, a contradiction in itself. He admits, further on, that by Phillips, in hall and bed-room, in winter nights "some of the light-giving properties of the gas are re-burning 5ft. per hour of gas, which was not carmoved," because the gas" cannot be purified without buretted. doing so." Other chemists differ from "Sigma" on this point: be it so or not, my words are confirmed by "Sig"himself, and while other towns are supplied with far richer gas, without dithculty 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 with such rich gas as Aberdeen and Glasgow.

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Your readers can also new judge as to "Sigma's" remarks about the price of London gas; he first told us it was the fault of the consumers; and now he admits that some companies pay a full dividend of from 7 to 10 per cent, and, not a bonus as I called it, but "arrears allowed

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 not a patent; and as I have given you my name, rank, and address, I feel confident you will not impute any improper motive in 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.

C. D. C.

the surfaces of an object-glass are
spherical, the spherical aberration lang
the concave form of the flint lens.
of my machine-to whic
working the lenses of an object-case
slightest doubt, and any object-gla
the same process that I have den.
the only difference in method bell -
should merely be a narrow
Strip
adhere to the surface of the objec
As to the query of A. White.
believe the drawback he would f
of good definition, which would b
able in his proposal as in the
telescope, and which, when using - •
be simply intolerable; such
however, be made just tolerable tys.
length of the speculum unusually loz.
and using only moderate powers.
of course, from the obliquity of the
rays with respect to the incident rats -
of which should exactly coincide with a
secure the best definition; this necessit
position of the plane, and this form, ez
the "Newtonian," undoubtedly
periority against all others.

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by law." In other words, the price is kept up to enable inquiry of your correspondent about Irish moss, I beg to map of the world, there is placed a figure el
the said companies to divide this excess of profit, to
cover their previous mismanagement and their own

"ruinous competitions!!"

As "

Sigma" admits Professor Frankland is not a 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 1867, as it will show your readers and "Sigma" that what has been said by me is not all "senseless gabble."

When dried it is

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READINGS FROM THE GLOBES.[266] SIR,-On the terrestrial globes, he.. equal distance on each side of the equator, and res far as 23 latitude, to this figure his be ez given Te of Analemma, though it is, in fact, nothing more f names of the twelve months with the number of de each month, and so arranged as to corre-poad L sun's place on each side of the equinoctial. This instance, the 21st March and the 20th it, are plave the equinoctial line, and come under that part of t brass meridian which is marked 0, and this serves i show that on those two days of the year the sun has declination. At the northern extremity of the Aralens is June 21st, and this is made to correspond with 21 North Declination, and in like manner the southern es tremity is marked December 214, and ewes ander

IRISH MOSS AND COD-LIVER OIL. [263] SIR,-As I see no one has yet replied to the inform him that it is a kind of sea-weed found on several parts of the coast of Ireland, where it is called carrigeen, druggist's under its English name. and it can be obtained at almost any respectable 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 Professor Frankland said that he had just had the and singlass, but it has the advantage over the former illuminating power of the gas supplied to different large of being nearly tasteless, and over the latter of having towns tested by the standard sperm candles, and then a specific action on the lungs, which makes it nearly if held written and signed certificates in his possession not quite equal in value to cod-liver oil. I have known of the results as follows:-Berlin, 155 candles; Paris, several cases where it alone has effected a cure after 123; London, 12.1; Vienna, 90; Edinburgh, 280; the patient had been supposed to be in hopeless de- S.D. The intermediate divisions or gradations being Manchester, 220; Liverpool, 22:0; Glasgow, 280; cline. The manner of using it is as follows:-Take made to correspond with the forest declinations of

by the gas furnished in London; but, in particular in

sulphurous acid and other gaseous vapours injurious to

Aberdeen, 350; Greenock, 255; Hawick, 300; Inver- about as much of the dry moss as will fill a common
the sun. Now this is a most insuous and easy method of
ness, 25'0; Paisley, 303; Carlisle, 160; Birmingham,
150. Thus the gas supplied to Edinburgh and Glas- teacup, and steep it overnight in cold water, which giving at a glance the explanation of many of the most
gow gives more than twice the light of the gas provided In the morning boil this just ten minutes in a quart of the uses of this figure are varied and most rap
removes most of the marine taste which belongs to it. important phenomena in mathematics jography, and
for London. The above shows the average light given milk, season with any flavour that is agreeable, and Numerous problems may be answerel instantaney,
when cold it will be found a strong jelly, like blanc- if we only know the principle on which the Anal
also, he stated, is richer than it ought to be in sulphur tuted. It may be made with water as well as with in what latitude he is vertical and on what days, 2
stances, it only equals nine candles. The gas of London mange, for which, if carefully made, it may be substi. is constructed; as, for instance, the sun's declin
compounds, and in burning gives off too much poisonous milk for variety, and is particularly beneficial if taken time of his rising and setting, the duration of twibghi,
in the morning (melted), about a teacupful at a time. and his meridian altitude; so that the Anal mais ze
health and property. London gas is now worse than it Its action in some cases, especially when there is of the most useful figures on the globe. In the prone
was many years ago, although the methods of manu- hemorrhage from the lungs, is wonderful. The ordi. instance we shall see its utility in finding the leorth
facture have been cheapened by the discoveries of
science, all new inventions in this direction having been nary price is about 3d. or 4d. an ounce, which, as it is the longest days in the frigid zones. Geographer
very light, is not expensive; but it is at least a dozen have found it convenient to divide this part into spaci
eagerly taken up by the gas companies, who, so far as times what it ought to be. A few years since, when I corresponding with the length of time the sun is ab
was living at Leeds, I was able to procure it at one shop the horizon-as, for instance, where the longest an
As the gas is damaging to for 4d. per lb., while the druggists were selling it of the one month long, where it is two months long, and » 2
until it reaches six months long. Now as the
fair price for it, at least have it of good quality. He respondent who wishes to find a substitute for cod-liver / shine over half the globe only at once, and as the esti
by the of the places in th

is known, have not adopted a single invention which
would benefit the consumer.
works of art and beauty, the public should, by paying a same quality at the same price per ounce.

power of twenty candles, below which no gas is fit for household use."

It is indeed much to be regretted that these scientific and interesting subjects are not discussed in a more courteous and liberal spirit, without indulging in personalities and dogmatic assertions and misquotations of each other's words and intentions.

Another correspondent takes objection to the experimental result I gave of the improved light obtained by the very crude apparatus described in No. 263, 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, &c., and 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 6s. 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 "Sigma's" remarks do not apply to such cases.

from it.

OUR SIZE AND FORM.

If your cor

G. BUSH,

[264] SIR,-While I agree with the "Harmonions Blacksmith" as to the probable enlargement of the ENGLISH MECHANIC at no late period, I differ from him as to the increase in price, 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 your 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 seen and read to be appreciated. Your

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.

zone, in the course of the year, will never go bel horizon at the North Pole six months, 234- from 18 month, and gradually increasing in the intern When the sn spaces till the pole is reached. declination, those within 5 of the pole will have d stant day; when 10 declination, those within 10 da pole will have constant day; consequently the res any place is to the pole the longer will the sun bea the horizon. To find, then, the number of days d which this is the case, we have only to look on the lemma, corresponding with the degree of the sa declination and the co-latitude, which, in this exam will be the same, and we shall find what we repa When his declination is high, the ecliptic and the treg of Cancer on the globe nearly coincide, so that the sp where the day is one month long is within a na strip measuring only 46', but the second strip is being 2 15', the strips gradually increasing in until the last, which is 7 wide. In Nova Zem whose latitude extends from 69° to 78°, the inhab of the most northernly part will begin to see this I must observe that what has been said by "Sigma" not taking any account of refraction, when his and also by "L. M." (p. 400, No. 66), about street nation reaches their co-latitude, that is 12, and loc lamps, does not apply to our domestic gas consumpon the Analemma, underneath 12°, we shall find the or lin tion. "Street lamps" require a very small and imwill see him from April 19th to August 28th, perfect carburetter to be fixed to each lamp, and not days; whereas the southern extremity, whose latitudes (as in a house) only one apparatus for all the burners. 69, will begin to see the sun above the horizon witho Dr. Letheby's official report on the experiment dissetting, when his declination is 21°. As, therefore, or belt tinctly states that the light was at first most excellent, [265] SIR,-Permit me to inform your correspon- during six months of the year these strips but gradually fell off, and caused great complaints, in dent Raymond," (4534, p. 527), that the plan he have a length of sunlight varying from one month consequence of which the attempt failed, as I stated in proposes for combining an object-glass would be quite to six months, so when the sun is in the opposite erroneous. A plano-convex lens of flint, in combi- declination they must have their nights varying To prove that this is not the case with such appa-nation with a crown lens of the same form, would only in like manner. But there is this to be con ratus as I have recommended, and which can be now tend to increase, and not correct, the chromatic dis-sidered with reference to the absence of the sun. obtained at moderate prices, I subjoin the results of persion, while the spherical aberration would remain that it cannot be actually called night, for as twiligh the last year's operation of the one I have used for almost the same. The theory of the achromatic object- lasts until the sun gets 18 below the horizon, and re years, which I designed and fitted up for my own glass consists in correcting the dispersive power of the fraction also raises a heavenly body when near the amusement and most decided advantage, for it only convex crown lens, by using a concave lens of higher horizon, it will be necessary to see how long it takes cost about 6s., and saves me fully 25s. a year, besides dispersive power but shallower curvature; and so the the sun to reach 18 below the horizon, and then w the advantages resulting in better air and less destruc- opposite tendency of one counteracts the opposite shall find that, although deprived of the sun's r tion to property. Not (as "Lucidus" wishes to make tendency of the other. If "Raymond" seriously these places are not without some compensation for out that I state) from any power of "purifying" in thinks of attempting an achromatic O. G., he had his absence-for twilight, refraction, the moon, and the apparatus, but simply such as is due to the reduced better examine one first, and seek to discover the the aurora borealis will do their best to impart some quantity of gas consumed to obtain the required reason for working the four surfaces to different kind of cheer to the inhabitants of these benighted power of light. I filled the carburetter with 1 gallons curvatures. objectionable on account of the greater difficulty of nation twilight will cease at the N.P., but this declinaPlano-convex lenses, when large, are regions. When, then, the sun reaches 18 south decliworking a plane than a curved surface. Again, the nation he will not reach till about the middle of Nor process of parabolizing is required only for reflectors, vember, and will reach it again before the end of

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ary, and during that interval of two months the - must necessarily be above the horizon some part 10 time, and if it affords no warmth it will at least light. The intermediate strips or belts, how. will never be totally deprived of twilight, bee only those places within 5 of the pole will be this state. ndary line where places can have no twilight all it is 48 north and south latitudes, and this is ertained in this way:-As the greatest declination 24, if we add to this 18, the extent of twilight, we 11 have 41 as the distance from the pole of all ces which enjoy this boon in the summer time; coninently, by subtracting this from 90', we shall have

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ISOMETRICAL.

from the want of practical farmers. Again, as a rule, it is not the 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 summer are also extremes? Is Western America a

[269] SIR,-I wish to draw the attention of "Isometros" 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 Again, in the temperate zones the the line A O in Fig. 2 measure lin., whereas the pro- panacea for all the evils under the sun? The prairie, I jection 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.

as the parallel where twilight all night ceasesat is, only those places whose latitudes exceed Quebec the western hemisphere, or Paris and Vienna in the stern, can have twilight all night. With respect to south polar regions, the poets of antiquity (see Georgics," Book I. line 244, &c.) could offer conjecires only, assigning to them the abodes of the inrnals and the presence of perpetual night; but odern science can offer reasons why the south polar egions should be enlightened during their night in the 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 fine poetry. T. S. H.

ame

J. K. P.

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 FIELD BOILER AND PRIMINGcompetency is to accept advice, gratuitously and sen[270] SIR,-Will some of my brother readers kindly tentiously given, but, unfortunately, lacking the most advise me in the following difficulty ?-Amongst essential quality of correctness. How does it happen, several of the same class, and of like and smaller sizes, if American railway prospectuses are so genuine, that I have a vertical semi-portable boiler, constructed on there exists so much fear regarding the flowery senthe "Field " principle, of the following dimensions:-tences they contain, and so many who state they have Shell, 11ft. 2in. high by 6ft. 7in. outside diameter; fire- been deceived by them? box (with water space all around of 6in.), 6ft. 2in. high by 5ft. 7in. diameter; 49 Field tubes 3ft. 7in. long, and 197 do. 3ft. 11 in. long, 24in. ontside diameter; total heating surface, 6914 square feet; nominal power of boiler, 50 horses; working steam pressure, 75lb. per square inch; depth of water over fire-box crown, 18in.; 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-circle 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 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 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 Can it be owing to the possible unfitness of to occur. feed water, and would heating saine to near boilingOr is steam point before using as feed affect the case? 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.

HEIGHT OF CLOUDS. [267] 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., the age and position of the moon, is wanting. The moon might have been so near the 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., 1889, in latitude 55° 45′ 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 say, 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 vapour 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.

HORSE POWER.

J. STEEL.

[268] SIR,-I should be 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 firetubes, 4ft. high, 1ft. Sin. in diameter; fire box 1ft. 5in. high (between furnace bars and crown of fire-box), by 1ft. 3in. diameter, 7 fire tubes 1ft. 1lin. long by 24in. outside diameter; water in boiler 10in. 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. Coals of good quality. Required:

(a) Quantity of steam of 60lbs. pressure (75 from a
vacuum) obtainable from this boiler per hour?
(b) If the tube surface above water line is as effec-
tive as that below in the way of producing steam?
And,
(c) If a greater quantity of steam would be made
and how much more by raising the water line to
15in, above crown of fire-box?

2. Required also to know, if for this quantity of steam (sub. 1 a) a non-condensing engine of 3in. diameter of cylinder and 7in. stroke, with common slide valve cutting off at (say) th 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 theoretic, and what the real or effective horse power 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 and highly-instructive journal, and hope, therefore, they will be forthcoming from those of your correspondents so well able to deal with investigations of this kind.

A. W. E.

P.S.-In my last

A WORKING ENGINEER.

query, No. 4513, I note mistakes on formula No. 1 should be explained thus:-n the theothe part of the printer. The algebraical signs of retic H.P. (i.e., horse power); the area of piston in square inches. Then, again, further on, R is used instead of H.P. (horse power). In query 3, H.P. (horse power) is also to be understood by R, 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 the place of "cylinder respectively," it should be read "cylinders respectively."

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THE NATURE AND CAUSE OF LIGHT.

[271] 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 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 the 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.

EMIGRATION.

T. A.

[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 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 mast, if not permanently, at least temporarily be filled up. Being impressed with these ideas, my intention was to refrain, for the present at least, from giving vent to thoughts upon the subjeet. 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," &c., and "this would in no way exclude such mechanics as smiths," If the advice given were followed, viz., &c. that a farmer, smith, and carpenter should emigrate in batches, I think emigration would soon be stopped

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. G.” certainly is extremely vague in the following sentence: -"He can see the lands himself and make his 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, &c., for whether satisfied or not, after the bargain is concluded he can make no further alteration? Although I feel inclined to criticise this extraordinaryletter 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 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 any one who writes upon this subject, I disagree with regarding the enormous responsibility resting upon 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.

[278] 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 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 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 intimidate others from learning the machine.

The bicycle club to which I belong always travels at the average of eight miles an hour for the whole we freWhen the road is down hill, journey. quently 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 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 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 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 an arithmetician. But I so unscrupulous from 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 a day's journey. I may mention for his comfort, that I could travel sixty miles 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 the "elaborate calculation "), it follows, of course, that to prove this theory he would require to walk one hundred

miles a day. I hope he 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 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. 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 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 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, 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 reality are, perfect in their adaptability to diversified circumstances. It is under particular circumstances 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 favour of the machine, because they happen 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 and 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 12 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 carrying 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.

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If any one buys a bicycle with the hope of, with ease 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 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 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 only 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 up a higher rate of speed than six or seven miles an hour

for any distance. I have frequently travelled eleven measured 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 seeing themselves immortalized in the papers; but performances of this kind 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 amount of vital energy, which would require a great deal of rest and food for its proper restoration.

If 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 "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 bicycle.

W.

results which acknowledged laws of electes a would have led me to expect

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I will state a few of the ideas which crowd an and inclined me to believe that, nota repeated failures, such a machine was constructed so as to give a useful ef sumption of fnel. My first observ motive force was the direct result of s and that for a definite amount d amount of motive force was pra always in such a way that man practical purposes. Now the electr being uppermost in my mind, I as Is e steam-engine an application of electri was, certainly. A coal fire is nothing than an electrical battery where the moment they are set free are reuniteds nothing more. Now this heat is the othe battery, and if we could utilize the t we should be utilizing the whole n battery consuming so many atoms of t by the weight of coal and oxygen from th every engineer knows how small an s useful power is obtainable. If we lock s. other form of battery with acids and met can apparently collect the whole effect d or chemical action, in two wires, we natant that if we had a machine which would use n we could give it from these wires and produces effect, we should have a machine which wedi cost less than a steam-engine, or rather w greater power in proportion to the weight of te.. sumed. Now I imagined, when I had over difficulty I mentioned before, I had a machiner would not be far from giving the desired resulte that by converting the high speed which ought given to a slower, but still useful speed, I shouli well. I have seen small wheels revolving w thousand times in a minute, but when I come to a dru whose diameter is 16in., and whose weight with fo wheel is 1001b., although it has a perfectly cratunvo power of at least 301b. acting on its circumference, it will not give more than 80 revolutions per minute with a battery of 40 cells (sulphuric acid only. Why is this? I cannot understand it.

MORDAN'S SELF-CENTERING WIRE CHUCK. [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 Mordan 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. I might state that by my arrangement the current The first one I made had not the plain part at C to has to make 1,600 trips on four separate wires siterguide the cap and keep it true, it is a great improve-nately when the drum makes 80 rendations. Th irons of the magnet poles are 3in. diameter.

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[276] 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 com. pletely, but am astonished to find that I do not get the

THINKER

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LIGHTNING CONDUCTORS. [277] SIR,-If you would grant me space, I it a duty to warn any querist very earnestly the quackery rife on this subject, and which als honest, but inadequately informed, writers to make such replies as the two in your current number (p.); the first wholly erroneous and mischieroas 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 inlan 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, is 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 protected, or si 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 surface," read sufficient bulk or capacity; and understand that a certain minimum area of section, varying with the nature d the metal, is necessary. That is to say, for safety every part of the conductor must be of greater s than the thickest piece of that same metal ever k to have been melted by lightning in this climate. Th is not the case with F. Bedford's "in. iron wink which would therefore be simply a source of danger, far worse than no conductor at all. That size is shor the minimum that could be held safe in pure copp which is six times more conductive than the best and twice as much so as commercial copper. real minimum of iron that should be allowed is s square inch, or of copper a third as much, or s quarter, if of very good quality. It is impossible, therefore, you see, for safe copper conductors to supplied, as "Q. Yorke" fancies, for 1s. a foot, though iron ones might be. And whatever might be the case, could we 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. Zine 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 part of the conductor mast be thinned or narrowed to less than the safe capacity. Where it is joined the pieces must be in flat contact over at least a square inch. If rounded, or touching only like the links of a chain, the first serious flash passing will fuse and destroy the contact, and the second will have to leap the interval by a fiery ex plosion. Mr. Walker's tapering steel "attractor" again will, the first time it is struck, be scattered in 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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