Abbildungen der Seite


HARRIS, ESQ., F. R. S. In the year 1825, Mons. Arago dis thick, it is made of fine steel, well temcovered the influence of metallic bodies pered and hardened throughout its whole in arresting the vibration of the magnetic length; it is placed edgeways, and is deneedle; he showed that the amplitude licately balanced on a fine point at C, of the arcs of vibration when the needle resting on a centre of agate. Instead of oscillated within a ring of metal, became the ordinary compass card, a transsensibly reduced, and the needle tended parent circular disc of talc, figs. 2 and rapidly to a state of rest; he further dis 3, is attached to the bar, at its under covered, that when a magnet was deli edge, on which the different points, &c., cately balanced on a fine centre, and are either painted in transparent colours, placed near the surface of a rapidly re or otherwise laid on it, on very thin volving metallic plate, it would be soon paper, so that the whole is quite transgreatly disturbed, would begin to oscil parent, and in order to ensure and relate, and would, if the motion of the gulate the horizontal position of the plate were sufficiently rapid, be dragged whole, at any time or place, there are two round with the plate.

small sliders of brass, d d, fig. 4, underThe results were further pursued and neath, and on each side of the centre, so investigated in this country by Sir John contrived as to slide and turn with friction Herschell, Mr. Babbage, and also by into any required direction by means of Mr. Harris, who repeated the experi two slits in them and small stop screws. ments in vacuo ; their researches led to

The compass bar with its talc circle the development of new facts of great beneath, is now placed centrally within a practical consequence.

ring of hammered or rolled copper as in Faraday, with a power of physical re fig. 5, the poles of the bar, which project search peculiar to himself, has since a little beyond the card, being distant shown that the force which thus fetters from the interior of the ring, about sth, and restrains the magnetic oscillation, is or 4th of an inch. This copper ring is not the result of ordinary magnetic ac about 11 inch wide, and fth thick ; it is tion, but is de dant on the generation turned up and finished in a lathe, so of electrical currents induced by the to be perfectly circular; the centre piece magnet whilst in motion, in the metallic M, fig. I, carrying the needle, is supriny; and which he terms Magneto Elec ported on a cross bar m n, fig. 5, attric Induction; no attractive power be tached to the ring, and the centre part c, tween the bodies being observable in a accurately adjusted in the lathe. state of rest.

The whole is finally set within a glass These facts having been fully esta bowl or other case, according as it is reblished, we are enabled to apply the ge quired to light the compass from beneath neral principle, with perfect safety, in or above, and placed in gimbles in the restraining the inconvenient oscillations usual way,—the perfect transparency of of the compass on ship board, so as to the single disc of talc renders the comconfine it as nearly as possible to its na pass card very visible and clear when tural direction, without in any way in lighted from beneath. terfering with its delicacy, or any liability The great steadiness of this compass to error from the restraining cause, and under all sorts of motion is very remarkthus avoid the disturbing motion, which able. It has been successfully tried in a often arises from the rolling and pitching few ships of the navy, and is still on of the ship

trial. The needle being placed on its This is the new feature in the com edge, it is liable to little error in its magpass invented by Mr. Snow Harris, and netic line; it has, besides, great magnetic which is now about to be described. energy and great delicacy of suspension,

The compass needle consists of a and is unembarrassed by a heavy cardstraight bar A B, fig. I, about 7 inches whilst at the same time, the magnetolong, an inch wide, and Jth of an inch electrical induction on the copper ring

effectually preserves its natural direction

undisturbed. These are advantages of no • Philosophical Transactions for 1825 and 1831, pts. 1 and 2.

ordinary kind.





Mr. Harris has shown (Transactions of the Royal Society for 1831,) that

Sir,-We may soon expect further when a magnetic bar oscillates freely evidence of “winter's icy hand" being within a series of concentric rings of

upon us ; and amongst the many shapes copper, accurately and closely fitted one in which this evidence will be afforded within the other, the restraining force of is, the annoyance to housekeepers by the the copper with a given magnet, is in freezing and bursting of water-pipes. versely as the squares of the distances

Some people look upon this as a nefrom the pole of the bar, and directly as

cessary consequence of frost; a sort of the quantity of copper within the sphere “compliment of the season," to be reof its action, the matter being supposed gularly expected, and by no means to be to be condensed into an indefinitely thin guarded against. Other persons desire ring, and taken at some intermediate, or to be freed from this annoyance, of which mean distance within the surface, where they have a very keen remembrance from the sum of the forces may be supposed previous visits; and, in order to provide to produce the same effect as if exerted

a remedy, they wrap a slight twist of from every part of the mass, and that hay, a few old rags, or a solitary piece of hence the energy is also directly as the

matting, round that portion of the servicedensity. He also found that with a given pipe which is exposed to the inclemency magnetic tension in the bar, the restrain of the weather. These matters are all ing force no longer increased sensibly

very good non-conductors, and would after a certain number of rings, that in effectually answer the purpose, if applied fact the number of rings requisite to in sufficient bulk; but it almost invariexhaust, as it were, the magnetic force, ably happens that this circumstance is varied in some ratio of the power of the either overlooked or misunderstood. In magnet ; thus with the bar employed, fact, the difficulty of surrounding the no sensible increase of energy in the pipe with a sufficient quantity of nonwhole was observable, after the tenth

conducting material amounts, in many ring, the effect being the same with ten

cases, almost to an impossibility. as with any greater number of rings. The plan which I have successfully

In the application therefore of a cop adopted is, to inclose the external portion per ring to the purpose of restraining of the water-pipe in a deal case or trough the oscillations of the compass at sea, it fitted to the wall, about four inches is desirable to have the poles of the bar

square, which is filled with finely-sifted as near as we can to the interior of the

coal ashes, which is a most excellent ring, to have the copper as dense as pos- non-conductor, and perfectly surrounds sible, and to give it greater or less thick

and encloses every portion of the pipe. ness, in proportion to the power of the

My water-pipe is in some degree shelcompass bar. Much has been said, and

tered, and for more exposed situations a many experiments tried, with a view of larger mass of ashes might be required; determining the best form for a compass but it is very easy to be on the safe side, needle ; it will, however, probably be by using a little excess. found, that the simple bar above de I have in a few instances seen Russell's described is, upon the whole, not only welded iron gas tubing used for external the most accurate, but in every respect service-pipes for water, and its adoption the best. Its form greatly simplifies the for this purpose, in all exposed situations, workmanship necessary to its construc would prevent the continual burstings tion, and admits of the various other

which severe weather always produces, scientific processes upon which its action and prove a great saving to the prodepends, being easily and perfectly car

prietor. ried out. If the steel be well chosen,

I remain, Sir, and be properly tempered, such a bar is

Yours respectfully, susceptible of a very high degree of mag

W. BADDELEY. netic energy, which it will be found to

29, Alfred-street, Islington. completely retain.- Transactions of the

December 27, 1841. Royal Cornwall Polytechnic Society.

[ocr errors]

HOOD ON HEAT. The members of the Institution of esteemed correspondent, (not Mr. WilCivil Engineers, having given an exten liams,) the following notes on the paper sive publicity, as well as a sort of implied in question by Dr. Kane, of Dublin ; sanction, to the contents of a paper lately which if they do not touch all its many read before that body, “On the Proper vulnerable points, will, at least, serve to ties and Chemical Constitution of Coal," give a tolerably correct notion of its geby a Mr. Charles Hood, abounding in neral character. The Mr. Hood, whose the grossest errors, theoretical and prac ignorance is here exposed, is the same tical, and which, if allowed to pass un person to whom the Council of the Innoticed and uncorrected, might obstruct stitution of Engineers, in some fit of somthe progress of those just views of the nolency, voted, about a year ago, one of subject, developed by our correspondent, their pretty medals for an Mr. Williams, in his admirable work On kindred worth, on warming and venCombustion, and further enforced in the tilation. Whether it be hot coals, or series of papers by that gentleman, now hot water, this gentleman meddles with, in course of publication in our journal — it seems to be equally his fate to burn we readily publish at the request of an his fingers.

essay of

Olefiant gas.

Notes by Dr. Kane, on Mr. Hood's Paper. 1st. The light carburetted hydrogen is and, without changing its volume, is connot, as asserted by Mr. Hood, among the verted into light carburetted hydrogen. first products of the distillation of coal; but. If it be frequently passed backwards and it is formed, on the contrary, only when forwards through the tube, it deposits all the volatile resin-oils and the olefiant gas its carbon, and the residual gas (the vo(which are, in reality, the first products) lume of which is doubled) is found to be are decomposed by sweeping over the pure hydrogen. The products of the ignited surface of coal, or metal of the distillation of coal may be arranged acretort, or its contents. When olefiant cording to the temperature at which they gas is passed through tubes heated to may be produced, as follows: bright redness it deposits half its carbon, 1st. Lowest temperature. Solids, as naphthaline, solid resins, and fluids, with

high boiling points. 2nd, or next temperature . Fluids which are very volatile. 3rd Stage 41h Stage

Light carburetted hydrogen gas. 5th, or highest temperature, Hydrogen gas.

In practice, however, the results of two merse the charcoal and iron, white hot, or three stages are always mixed toge into a mixture of light carburetted hyther.

drogen without any danger. The whole 2d. Light carburetted hydrogen is more use of the safety lamp depends on this, difficult to inflame than olefiant gas (Mr. 4th. Mr. Hood is quite in error respectHood's

's paper states the reverse.) Davy ing the source of the ascensional power has fully proved this; and I have verified of gas and its law; it has nothing to do his result, that a mixture of air and ole with the law of tranquil diffusion into fiant gas will explode at a temperature space, with which he has confounded it. that will not produce action on a mixture 5th. He is also wrong respecting the of air and light carburetted hydrogen. source of the great heating powers of the

3rd. The heat produced by olefiant gas, resin fuel. The idea of an increased in burning, is greater than that produced draught from the quantity of vapour by the combustion of the same volume of formed is also quite incorrect. light carburetted hydrogen in the propor 6th. There is nothing gained by the tion of 27 to 18. The weights are then, production of a gas requiring less oxygen however, as their specific gravities—that (as Mr. Hood supposes) than olefiant gas is, as 98 is to 56. If we plunge a piece does, for there would then be less heat of bright red charcoal, or a bright red produced. The quantity of heat evolved iron rod, into a mixture of olefiant gas in the burning of any body is propor. ml air, it will explode; but we may im tional to the quantity of oxygen absorbed,


21 and it is hence the interest of the opera 32' to 212, no idea can be thereby tor to use as much oxygen as possible, formed of the quantity of heat evolved. instead of the reverse. With regard to But if, in another trial, it be found, that the law of the quantity of heat evolved the burning of a pound of charcoal raises being proportioned to the quantity of oxy the temperature of 74lbs. of water through gen consumed, the following extract the same range, it follows, that the charfrom the article “Combustion," in my coal has double the calorific power of the Elements of Chemistry, (now in the wood. True relative numbers can thus press,) will be sufficient to explain it. be obtained, although they have, inde

* The determination of the quantity of pendently, no positive signification. The heat produced during the combustion of results obtained in this way, by various a given quantity of a combustible sub- experimentors, have been exceedingly stance is a problem of great importance discordant; but, by the late researches in the arts, as on it depends the economic of Despretz and of Bull, a very interestvalue of all varieties of fuel. The plan ing rule has been obtained. It is, that generally followed has been to burn the in all cases of combustion the quantity of substance, by means of the smallest quan heat evolved is proportional to the quantity of air which is sufficient, in a vessel tity of oxygen which enters into combisurrounded, as far as possible, with wa nation. Thus Despretz found that there ter. If it be found that the burning a are heated from 32° to 212° by pound of wood heals 37lbs. of water from

1 lb. of oxygen, uniting with hydrogen, 29} lbs. of water.

charcoal, 29


28 1,

ether, giving, as a mean, 283lbs. as the quan ever, is liable to

some very curious tity of water heated from 32° to 212° by changes,” &c. &c. the heat evolved in the combination of onc pound of oxygen. This rule, how Laboratory of the Apothecaries' Hall, Dublin,


[ocr errors]


AND CRANK ENGINES. BY MR. W. RADLEY C. E. Sir,–Observing in a recent number of set, I could not produce half the effect your Journal a requisition by yourself for obtained when the arrest of impetus took some engineer to furnish your pages with place at half the stroke. The reason of an exposition of the discrepancy of re this will be obvious to your readers, sult between the Cornish lifting and crank without any elaborate explanation. engines, together with an explanation Some little time after this, whilst obshowing in what consists the difference serving the motions of a lifting engine, of power elicited, under similar circum on the Cornish plan, which has a 10-feet stances, by those two modifications of the stroke, I became acquainted with the steam-engine, I take the liberty of com fact, that the motion of the piston was municating to you some ideas on the two-fold; that is, the down-stroke was a subject, and also some experimental data, separate and distinct function, perof which some were suggestive of, and fect in itself, and the up-stroke was others suggested by, these ideas. merely a preparation for the former,

Some years ago I conceived the notion and had otherwise no connection with it. of converting the momentum of the Pursuing my observations a little farther, steam-driven piston to the purpose of I found that this engine made nearly six die-sinking for calico printers, by affixing strokes per minute, and that each coma stuffing-box to the cylinder bottom, for plete movement occupied, as near as may the passage of a punch-socket attached to be, 10 seconds; of these 10 seconds, 5 the piston ; and I found that when the seconds were occupied by the pause, motion of the piston was regulated by a rather more than 2į seconds by the upcrank and fly-wheel, and allowed to de stroke, and the remainder by the downscend through ths of its stroke, ere stroke. coming by means of the punch to a dead From these facts it follows, of neces

sity, that the calculations of Mr. Pilbrow 7 = 157.5, we obtain a theoretical result and all others upon this subject must be much in accordance with the absolute duty. erroneous; and that in the instance of

The higher ratio of the Cornish lift may the Wheal Vor Borlase's engine, quoted be safely ascribed to the difference of by Mr. Pilbrow, (No. 947,) instead of friction in the two cylinders, one being the mean velocity of the piston being 84, the other 32 inches; coupled with stated at 10 x 567 = 56.70 x 2 = 113.40 the difference in number of strokes be

60 feet per minute, it ought to be 10 x

tween the two engines.

24 I will now subjoin a tabulated view of - 27 270 feet per minute.

Now, if a comparison betwixt the two engines; We compare

this with the developed only supposing, in order to aid the compower of the Cornish crank engine, also parison, that the crank had the same quoted by Mr. Pilbrow, whose strokes stroke as the lifting engine, viz., 10 feet, were 9 feet each, and 8.8 of them in the and each 6 strokes per second. I have minute, making about 150 feet per mi. divided the down-stroke, in each case, nute, (which Mr. Pilbrow calculates to into 10 equal parts of the crank pin's be only about ths of the duty of the gyration, equal to one foot of the piston's


descent. lifting engine,) by taking


22:5 x

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][ocr errors][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small]

By this table it will be seen, that neither of the two pistons moves through equal spaces in equal times; and that not only is this discrepancy greatest in the crank-guided piston, but that the unguided piston has a higher mean speed.

I will not vouch for the absolute accuracy of all I have bere set forth ; but, as it is the principle which forms my theme, I crave the indulgence of your liberal and better informed readers.

On what ground Mr. Pilbrow can, under these eircumstances, claim any superiority for his engine I am at a loss to conjecture; but I would, before concluding, inquire what is the cause, nature, and real mode of operation of the pause in the lifting engine? I think it can be easily shown, when the cause is

considered, that this function, or absence of function, can contribute nothing to the efficiency of the engine. For, let us suppose there is a vacuum in the condenser, equal to 27 inches of mercury, and that the cylinder at the top of the stroke is filled with steam of 6 inches of mercury; in that case, if the plunge-weight, which is to balance and overcome these forces, is inadequate to the task, then the valve between the cylinder and condenser will not open, and this it is which is the occasion of the pause. As the su am in the cylinder, by the tendency to an equation of temperature, becomes attenuated, the forces productive of the pause give way, the valve opens, and at that instant the piston is in rapid motion. What has been taking place in the condenser in the

« ZurückWeiter »