DESCRIPTION OF A NEW MARINER'S COMPASS, INVENTED BY W. SNOW In the year 1825, Mons. Arago discovered the influence of metallic bodies in arresting the vibration of the magnetic needle; he showed that the amplitude of the arcs of vibration when the needle oscillated within a ring of metal, became sensibly reduced, and the needle tended rapidly to a state of rest; he further discovered, that when a magnet was delicately balanced on a fine centre, and placed near the surface of a rapidly revolving metallic plate, it would be soon greatly disturbed, would begin to oscillate, and would, if the motion of the plate were sufficiently rapid, be dragged round with the plate. The results were further pursued and investigated in this country by Sir John Herschell, Mr. Babbage, and also by Mr. Harris, who repeated the experiments in vacuo; their researches led to the development of new facts of great practical consequence.* Faraday, with a power of physical research peculiar to himself, has since shown that the force which thus fetters and restrains the magnetic oscillation, is not the result of ordinary magnetic action, but is dependant on the generation of electrical currents induced by the magnet whilst in motion, in the metallic ring; and which he terms Magneto Electric Induction; no attractive power between the bodies being observable in a state of rest. These facts having been fully established, we are enabled to apply the general principle, with perfect safety, in restraining the inconvenient oscillations of the compass on ship board, so as to confine it as nearly as possible to its natural direction, without in any way interfering with its delicacy, or any liability to error from the restraining cause, and thus avoid the disturbing motion, which often arises from the rolling and pitching of the ship. This is the new feature in the compass invented by Mr. Snow Harris, and which is now about to be described. The compass needle consists of a straight bar A B, fig. 1, about 7 inches long, an inch wide, and th of an inch Philosophical Transactions for 1825 and 1831, pts. 1 and 2. thick, it is made of fine steel, well tempered and hardened throughout its whole length; it is placed edgeways, and is delicately balanced on a fine point at C, resting on a centre of agate. Instead of the ordinary compass card, a transparent circular disc of talc, figs. 2 and 3, is attached to the bar, at its under edge, on which the different points, &c., are either painted in transparent colours, or otherwise laid on it, on very thin paper, so that the whole is quite transparent, and in order to ensure and regulate the horizontal position of the whole, at any time or place, there are two small sliders of brass, d d, fig. 4, underneath, and on each side of the centre, so contrived as to slide and turn with friction into any required direction by means of two slits in them and small stop screws. The compass bar with its tale circle beneath, is now placed centrally within a ring of hammered or rolled copper as in fig. 5, the poles of the bar, which project a little beyond the card, being distant from the interior of the ring, about th, or 4th of an inch. This copper ring is about 1 inch wide, and th thick; it is turned up and finished in a lathe, so as to be perfectly circular; the centre piece M, fig. 1, carrying the needle, is supported on a cross bar m n, fig. 5, attached to the ring, and the centre part c, accurately adjusted in the lathe. The whole is finally set within a glass bowl or other case, according as it is required to light the compass from beneath or above, and placed in gimbles in the usual way, the perfect transparency of the single disc of talc renders the compass card very visible and clear when lighted from beneath. The great steadiness of this compass under all sorts of motion is very remarkable. It has been successfully tried in a few ships of the navy, and is still on trial. The needle being placed on its edge, it is liable to little error in its magnetic line; it has, besides, great magnetic energy and great delicacy of suspension, and is unembarrassed by a heavy cardwhilst at the same time, the magnetoelectrical induction on the copper ring effectually preserves its natural direction undisturbed. These are advantages of no ordinary kind. FREEZING OF WATER PIPES. Mr. Harris has shown (Transactions of the Royal Society for 1831,) that when a magnetic bar oscillates freely within a series of concentric rings of copper, accurately and closely fitted one within the other, the restraining force of the copper with a given magnet, is inversely as the squares of the distances from the pole of the bar, and directly as the quantity of copper within the sphere of its action, the matter being supposed to be condensed into an indefinitely thin ring, and taken at some intermediate, or mean distance within the surface, where the sum of the forces may be supposed to produce the same effect as if exerted from every part of the mass, and that hence the energy is also directly as the density. He also found that with a given magnetic tension in the bar, the restraining force no longer increased sensibly after a certain number of rings, that in fact the number of rings requisite to exhaust, as it were, the magnetic force, varied in some ratio of the power of the magnet; thus with the bar employed, no sensible increase of energy in the whole was observable, after the tenth ring, the effect being the same with ten as with any greater number of rings. In the application therefore of a copper ring to the purpose of restraining the oscillations of the compass at sea, it is desirable to have the poles of the bar as near as we can to the interior of the ring, to have the copper as dense as possible, and to give it greater or less thickness, in proportion to the power of the compass bar. Much has been said, and many experiments tried, with a view of determining the best form for a compass needle; it will, however, probably be found, that the simple bar above dedescribed is, upon the whole, not only the most accurate, but in every respect the best. Its form greatly simplifies the workmanship necessary to its construction, and admits of the various other scientific processes upon which its action depends, being easily and perfectly carried out. If the steel be well chosen, and be properly tempered, such a bar is susceptible of a very high degree of magnetic energy, which it will be found to completely retain.-Transactions of the Royal Cornwall Polytechnic Society. FREEZING OF WATER PIPES. 19 Sir, We may soon expect further evidence of "winter's icy hand" being upon us; and amongst the many shapes in which this evidence will be afforded is, the annoyance to housekeepers by the freezing and bursting of water-pipes. Some people look upon this as a necessary consequence of frost; a sort of "compliment of the season,' to be regularly expected, and by no means to be guarded against. Other persons desire to be freed from this annoyance, of which they have a very keen remembrance from previous visits; and, in order to provide a remedy, they wrap a slight twist of hay, a few old rags, or a solitary piece of matting, round that portion of the servicepipe which is exposed to the inclemency of the weather. These matters are all very good non-conductors, and would effectually answer the purpose, if applied in sufficient bulk; but it almost invariably happens that this circumstance is either overlooked or misunderstood. fact, the difficulty of surrounding the pipe with a sufficient quantity of nonconducting material amounts, in many cases, almost to an impossibility. In The plan which I have successfully adopted is, to inclose the external portion of the water-pipe in a deal case or trough fitted to the wall, about four inches square, which is filled with finely-sifted coal ashes, which is a most excellent non-conductor, and perfectly surrounds and encloses every portion of the pipe. My water-pipe is in some degree sheltered, and for more exposed situations a larger mass of ashes might be required; but it is very easy to be on the safe side, by using a little excess. I have in a few instances seen Russell's welded iron gas tubing used for external service-pipes for water, and its adoption for this purpose, in all exposed situations, would prevent the continual burstings which severe weather always produces, and prove a great saving to the proprietor. I remain, Sir, 29, Alfred-street, Islington. December 27, 1841. W. BADDELEY. HOOD ON HEAT. The members of the Institution of Civil Engineers, having given an extensive publicity, as well as a sort of implied sanction, to the contents of a paper lately read before that body, "On the Properties and Chemical Constitution of Coal," by a Mr. Charles Hood, abounding in the grossest errors, theoretical and practical, and which, if allowed to pass unnoticed and uncorrected, might obstruct the progress of those just views of the subject, developed by our correspondent, Mr. Williams, in his admirable work On Combustion, and further enforced in the series of papers by that gentleman, now in course of publication in our journalwe readily publish at the request of an Notes by Dr. Kane, 1st. The light carburetted hydrogen is not, as asserted by Mr. Hood, among the first products of the distillation of coal; but. it is formed, on the contrary, only when the volatile resin-oils and the olefiant gas (which are, in reality, the first products,) are decomposed by sweeping over the ignited surface of coal, or metal of the retort, or its contents. When olefiant gas is passed through tubes heated to bright redness it deposits half its carbon, 1st. Lowest temperature. 2nd, or next temperature. esteemed correspondent, (not Mr. Wil- on Mr. Hood's Paper. and, without changing its volume, is converted into light carburetted hydrogen. If it be frequently passed backwards and forwards through the tube, it deposits all its carbon, and the residual gas (the volume of which is doubled) is found to be pure hydrogen. The products of the distillation of coal may be arranged according to the temperature at which they may be produced, as follows: Solids, as naphthaline, solid resins, and fluids, with Fluids which are very volatile. Light carburetted hydrogen gas. 5th, or highest temperature, Hydrogen gas. In practice, however, the results of two or three stages are always mixed together. 2d. Light carburetted hydrogen is more difficult to inflame than olefiant gas (Mr. Hood's paper states the reverse.) Davy has fully proved this; and I have verified his result, that a mixture of air and olefiant gas will explode at a temperature that will not produce action on a mixture of air and light carburetted hydrogen. 3rd. The heat produced by olefiant gas, in burning, is greater than that produced by the combustion of the same volume of light carburetted hydrogen in the proportion of 27 to 18. The weights are then, however, as their specific gravities-that is, as 98 is to 56. If we plunge a piece of bright red charcoal, or a bright red iron rod, into a mixture of olefiant gas and air, it will explode; but we may im merse the charcoal and iron, white hot, into a mixture of light carburetted hydrogen without any danger. The whole use of the safety lamp depends on this. 4th. Mr. Hood is quite in error respecting the source of the ascensional power of gas and its law; it has nothing to do with the law of tranquil diffusion into space, with which he has confounded it. 5th. He is also wrong respecting the source of the great heating powers of the resin fuel. The idea of an increased draught from the quantity of vapour formed is also quite incorrect. 6th. There is nothing gained by the production of a gas requiring less oxygen (as Mr. Hood supposes) than olefiant gas does, for there would then be less heat produced. The quantity of heat evolved in the burning of any body is proportional to the quantity of oxygen absorbed, HOOD ON HEAT. and it is hence the interest of the operator to use as much oxygen as possible, instead of the reverse. With regard to the law of the quantity of heat evolved being proportioned to the quantity of oxygen consumed, the following extract from the article "Combustion," in my Elements of Chemistry, (now in the press,) will be sufficient to explain it. The determination of the quantity of heat produced during the combustion of a given quantity of a combustible substance is a problem of great importance in the arts, as on it depends the economic value of all varieties of fuel. The plan generally followed has been to burn the substance, by means of the smallest quantity of air which is sufficient, in a vessel surrounded, as far as possible, with water. If it be found that the burning a pound of wood heats 37lbs. of water from 1 lb. of oxygen, uniting with 1 giving, as a mean, 28 lbs. as the quantity of water heated from 32° to 212° by the heat evolved in the combination of one pound of oxygen. This rule, how 21 32 to 212, no idea can be thereby formed of the quantity of heat evolved. But if, in another trial, it be found, that the burning of a pound of charcoal raises the temperature of 74lbs. of water through the same range, it follows, that the charcoal has double the calorific power of the wood. True relative numbers can thus be obtained, although they have, independently, no positive signification. The results obtained in this way, by various experimentors, have been exceedingly discordant; but, by the late researches of Despretz and of Bull, a very interesting rule has been obtained. It is, that in all cases of combustion the quantity of heat evolved is proportional to the quantity of oxygen which enters into combination. Thus Despretz found that there are heated from 32° to 212° by INQUIRY INTO THE CAUSES OF THE DIFFERENCE BETWEEN THE CORNISH LIFTING AND CRANK ENGINES. Sir,-Observing in a recent number of your Journal a requisition by yourself for some engineer to furnish your pages with an exposition of the discrepancy of result between the Cornish lifting and crank engines, together with an explanation showing in what consists the difference of power elicited, under similar circumstances, by those two modifications of the steam-engine, I take the liberty of communicating to you some ideas on the subject, and also some experimental data, of which some were suggestive of, and others suggested by, these ideas. Some years ago I conceived the notion of converting the momentum of the steam-driven piston to the purpose of die-sinking for calico printers, by affixing a stuffing-box to the cylinder bottom, for the passage of a punch-socket attached to the piston; and I found that when the motion of the piston was regulated by a crank and fly-wheel, and allowed to descend through ths of its stroke, ere coming by means of the punch to a dead BY MR. W. RADLEY C. E. set, I could not produce half the effect obtained when the arrest of impetus took place at half the stroke. The reason of this will be obvious to your readers, without any elaborate explanation. Some little time after this, whilst observing the motions of a lifting engine, on the Cornish plan, which has a 10-feet stroke, I became acquainted with the fact, that the motion of the piston was two-fold; that is, the down-stroke was a separate and distinct function, perfect in itself, and the up-stroke was merely a preparation for the former, and had otherwise no connection with it. Pursuing my observations a little farther, I found that this engine made nearly six strokes per minute, and that each complete movement occupied, as near as may be, 10 seconds; of these 10 seconds, 5 seconds were occupied by the pause, rather more than 24 seconds by the upstroke, and the remainder by the downstroke. From these facts it follows, of neces 7 = 157.5, we obtain a theoretical result much in accordance with the absolute duty. The higher ratio of the Cornish lift may be safely ascribed to the difference of friction in the two cylinders, one being 84, the other 32 inches; coupled with the difference in number of strokes between the two engines. I will now subjoin a tabulated view of a comparison betwixt the two engines; only supposing, in order to aid the cornparison, that the crank had the same stroke as the lifting engine, viz., 10 feet, and each 6 strokes per second. I have divided the down-stroke, in each case, into 10 equal parts of the crank pin's gyration, equal to one foot of the piston's descent. 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 here 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 steam 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 |