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INTERCEPTION OF FLAME BY MESHED

FABRICS.

Sir, I am disposed to believe, that when Sir Humphrey Davy discovered the peculiar property of wire-gauze to withstand the passage of flame, he was not aware that combustible and nonconducting substances, in a similar state, have a like power. In some recent experiments, I have found that flame will not pass through the apertures in card, paper, lace, &c., even when the apertures are of considerable magnitude, which would go to prove that the principle laid down by Sir Humphrey Davy that flame cannot pass through apertures of small diameter-is really a universal law.

I have some difficulty in reconciling the above facts with Davy's explanation of the phenomena. He said the flame was prevented from passing through the wire-gauze in consequence of its being a good conductor of heat, and therefore cooling the flame below the point of kindling. As the wire soon becomes red hot, it is difficult to understand the cooling effect; but it becomes still more difficult of explanation when non-conducting substances are employed, which abstract little or no heat, and yet most effectually ⚫ oppose the passage of flaine.

From a consideration of these facts, it would almost appear that the ultimate atoms of flame are of considerable determinate magnitude, and also that they are held together by a cohesive force of such energy as to prevent their being easily divided.

Is the smallest body of flame noticed, as in the combustion of fine thread, composed of more than one atom? Because there appears to me to be some relation or correspondence between the size of such flame, and of the apertures capable of excluding flame.

I remain, yours respectfully,
WILLIAM BADDELEY.

London, August 5, 1834.

[We subjoin an extract from a very ingenious and interesting pamphlet, by Mr. John Murray, F. S. A., F. L. S., published 1833, entitled "Practical Observations on the Phenomena of Flame and Safety Lamps," from which our esteemed correspondent Mr. Baddeley will perceive, that the fact of flame being intercepted by other meshed fabrics as well as wire gauze, has been before this matter of frequent observation. Mr. Murray complains that his experiments on this subject, though long ago made known to the world, have not been appreciated as they deserve by scientific men. And well he may, since ap

parently they have been slighted for no other reason than that they happened to be at variance with the doctrines of the great scientific authority of the past age-Sir Humphrey Davy, although these doctrines, as far as regards the matter in hand, are now almost universally admitted to have been extremely fallacious.-ED. M. M.1

He,

"The wire gauze can be supposed to cool the flame only in two ways-either by radiation or conduction. Though it should be granted, causa argumenti, to wire gauze at the common temperature,' it is not quite so evident how red hot wire gauze could cool down flame, and intercept its passage, though I have had the safety lamp red hot in the passages of the mine where an explosive atmosphere prevailed. In a very early stage of this inquiry, I found that sieves of pasteboard, haircloth, and muslin, intercepted flame; and in 1818 proved these facts experimentally, in my public lectures at the Surrey Institution. These being non-conductors of caloric, it is evident that the question is reduced to the alternative of radia tion, and this in them is not sufficiently powerful to account for the phenomenon. Since that period I had a net-work tissue of transparent glass, and found that this, in like manner, intercepted flame. I dipped a piece of muslin into a solution of phosphate of ammonia, and having dried it, formed it into a safety lamp, when it was proved to be as safe, when immersed into an explosive atmo sphere, as one of wire gauze. Signor Libri, of Florence, accounts for the phenomenon of the safety lamp on principles altogether different from those of Sir H. Davy. Flame, he observes, is repelled by metallic surfaces, and that to a certain extent; if two parallel wires be brought within a specific distance, this repulsive void will be maintained, and flame will not pass through. therefore, finds that parallel wires will as effec tually intercept flame as when woven into meshes. Mr.Dillon's (of Belfast) improvement on the safety lamp consists of a shield of tale, forming an external semi-circular attachment, and its object is to PREVENT the cooling of the lamp by screening it from the current of air which flows through the passages of the mine, in the process of ventilation; he has thus shown that gas may be safely burnt in the lamp. It seems, therefore, evident that the principle of safety cannot depend on any cooling influence whatever, whether it be connected with conduction or radiation. Accordingly it is found that when the safety lamp is newly lighted, and is consequently more cool than at other times, it is very apt to explode the air of the mine, and will kindle a stream of hydro-carbonate, under such circumstances, even when six folds of wire gauze interpose, while it will not permeate ONE such fold if the lamp be нот. The rarefaction or attenuation of the inflammable and explosive atmosphere seems to be the real truth of the case.t When an explosive atmosphere is condensed, its explosive powers are more forcible, and its range more extensively destructive; on the other hand, when it is rarefied or attenuated, its violence is proportionably diminished. The heated walls of the wire gauze which imprisons the flame will attenuate the elements of explosion, and they will become feeble in consequence. The Chevalier Aldini's fire-proof apparatus, recently introduced, renders altogether problematical, if not entirely nugatory, the cooling theory, though the repeated objections we brought against it, from the period when the proposition was first started, had been entirely disregarded. Atdini's experiments made

"It providentially happens that the firedamp' of mines is among the least inflammable of the gases, or, more correctly speaking, the least susceptible of inflammation."

+"I purposely overlook the experiments of Von Grotthus, as not affecting this question."

at Geneva, and since repeated at the Royal Institution, seem to have startled the advocates of the theory referred to, and seeing that cloth of amianthus is a non-conductor, cooling by conduction, the note that was struck in past times, is abandoned, and its cooling influence as a radiator is now contended for. The latter is just as powerless as the former. Why should Chevalier Aldini's experiments, proving that flame would not pass through the meshes of amianthus cloth, make those who contend for the cooling theory, more restless than my experiments with meshes of hair cloth, pasteboard, &c., publicly exhibited in 1818? Their nature is precisely the same-they are equally averse to the theory that has been indulged in, and so long clung to with remarkable tenacity,"

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tive than a single ring of the same weight. Indeed, in the making of artificial magnets this principle of having a large surface of metal is attended to, as they are generally made of iron, a cross section of which is oblong. There can be little doubt, if experiments on a large scale, in this branch of philosophy, were made, they would lead to results of a very important description.

I remain, Sir,

Your humble servant, MENTOR.

OBSERVATIONS

ON THE MAGNETO-
ELECTRIC RING.

Sir, I hear that attempts have been made to realise the idea of your ingenious correspondent p. μ., since the important discovery was made, that electricity can be developed and chemical experiments performed by means of magnetism, but as yet with indifferent success. Indeed no experiments on a sufficiently large scale bave been attempted. For making experiments on electro-magnetism on a large scale, a circumstance of much importance has not been attended to, and has been overlooked by your correspondent. It is well known that the larger the surface of iron which is presented to the wires, so the greater will be the intensity of magnetism developed, as the magnetism is known to be confined to the surface of metals. If we get two pieces of similar form, or two rings, one of which will be double the weight of the other, the proportion between the surface of these rings will be considerably less; the consequence is, that with a large ring there cannot be a sufficient quantity of wire coiled on it without putting it on in too many thicknesses, so that the outer coils are too far removed from the iron. Four thicknesses at the utmost is the most that can be effective. The celebrated American magnet (electro) was made of iron 2 inches square. This presented a greater surface, weight for weight, than 'round iron. I would beg leave to suggest to your correspondent, to make the large ring or rings of iron 1 inch in thickness, and 6 or 9 inches in breadth, bent in the form of the iron rim of a wheel. Two or more of these rings, for the same reason, would be more effec

Sir, The above extract from a doggrel advertisement of the celebrated omnibus-man, refers to an opinion, very prevalent of late among unscientific roadsters, that wheels with convex or semicircular tires run lighter than those equipped with tires of the usual flat cylindrical form.

If our roads were infinitely smooth and hard, no doubt wheels of the form in question would run very light; but in the present state of roads-good as they now are-it must be evident to all who carefully consider the matter, that cylindrical faced wheels are best.

The round-tire wheels cut into the road much deeper than flat ones, and have a greater hill constantly before them.

The omnibus in question is not the only vehicle that is equipped in the manner stated; several stage-coaches, and some of the mails, are furnished with round-tire wheels. On looking at an Exeter mail the other day, I observed that one of the hind-wheels, with round tire, had been locked and dragged down a hill, which had ground the tire quite flat in that particular place, and this must always occur under like circumstances, from the small quantity of surface supporting the weight and exposed to the grinding action. By the use of a shoe, however, this could easily be guarded against, but there are many other objections not so soon obviated.

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The question of broad versus narrow wheels, has long since been disposed of pretty satisfactorily; but its application, in point of principle, to the use of flat tires in preference to round ones, does not appear so plain to modern wheelwrights as it should do.

I am, Sir, yours respectfully,
WILLIAM BADDELEY.

London, August 6, 1834.

A FEW WORDS IN BEHALF OF
MATHEMATICAL SCIENCE.

Sir,-Will you permit a foreigner (a Swiss) to make a few remarks on Mr. Cheverton's article on the value and importance of mathematical science.

In the first place I must state, that had it not been for the false aspersions he has heaped upon some of the most illustrious of my countrymen, and more particularly on that excellent man Euler, I should have left him to enjoy all the honours and benefits that may arise to him from the said article.

In the first observation he makes upon Euler (No. 572, p. 277), he says, "the most valuable method of determining the longitude was at last accomplished through practical means and by practical men. Euler aimed to produce an achromatic lens as well as Dollond, but it was the latter who succeeded; and their respective modes of proceeding mark both the character and comparative value of their efforts."

Well, in all this there is, perhaps, nothing very extraordinary (upon the supposition that Mr. Cheverton's statement is true). Euler belonged to that useless race of men called mathematicians (according to the Chevertonian philosophy), and Mr. Dollond (Mr. John Dollond I presume he means) was certainly a first-rate optician; not only so, but also (much to his discredit, no doubt) a clever mathematician and an able astronomer.

Mr. James Gregory, professor of mathematics in St. Andrew's, as it appears from his "Optica Promota," showed how a telescope with specula might be constructed; but this unfortunate mathematician being neither an artist, nor having any turn for practical mechanics, was himself incapable of realising his

own invention by an actual construction, although he had demonstrated that to form a perfect image of the object the speculum must have the form of a parabola. I might here, Mr. Editor, pause to inquire, whether the arts or sciences at this period were in advance ; but the question would carry its answer along with it. The unfortunate Gregory could not find a single artist in his own country that could grind a speculum for him. Poor Scotland! this appears to be a stain upon your escutcheon. But why, it may be asked, did not Gregory apply to some of the English artists? The answer is easily given. They were not, at this period, one jot farther advanced in the arts than their northern neighbours.

Soon after the period just alluded to, the immortal Newton found himself compelled to relinquish for a time his philosophical pursuits, and betake himself to the mechanical art of grinding specula. His first attempt was to construct a reflecting telescope, according to the principles of his friend Gregory, that is, with a parabolic speculum. In this, however, he did not succeed; but he succeeded in making two reflectors with his own hands, which he presented to the Royal Society in 1672, the great speculums of both being of a spherical figure, although he was fully aware of the superiority of the parabolic form. During the interval he was employed in making his reflectors, he tried his hands upon constructing a refracting telescope, which had been prepared by Descartes, the object of which was to grind the lenses into one of the figures of a conic section; and when employed in this pursuit he made his grand discovery of the various refrangibility of the rays of light, from which he concluded, that the errors arising from the spherical figure of the lens were small in comparison with those which arose from refrangibility. He, therefore, gave up the attempt of constructing refracting telescopes, considering that the devergency of the rays of light, arising from the difference of their refrangibility, was always in proportion to the refracting powers of the medium. In consequence of this opinion of Sir Isaac Newton, respecting refracting telescopes, all attempts to construct them free from the effects of refrangibility were, for a pe