« PreviousContinue »
count of the machine invented, or at least conftructed by him, for the purpose of producing an intense heat, at confiderable diftances, by means of the folar rays reflected from a great number of plain mirrors, fo difpofed as to throw numerous images of the fun on the fame spot. In the first part of this memoir, and in the fucceeding article, which contains his subsequent obfervations, and answers to the objections made to fome parts of his doctrine by certain philofophers, the Author explains the theory on which this invention was founded, and which is not fo obvious as it appears to be at first fight; as according to the doctrine of Des Cartes and fome other optical writers, the fuccefs must have been impoffible. He fhews that this was the only method by which the fun's rays could be made to produce an intense heat at great diftances; for that, even supposing that it were practicable to form and grind a concave mirror of a very large fize, on a fphere of 600 feet in diameter, for inftance-in which cafe its focus would be at the distance of 150 feet-fuch a mirror, the conftruction of which however is impracticable, would not have any confiderable advantage over a combination of plain mirrors, of proper fizes, and having the fame extent of furface. According to his computation, the advantage of the most perfect concave mirror over the abovementioned combined plain mirrors would only be in the ratio of 17 to 10 nearly.
On this part of the fubject we fhall only obferve that the diminution of the effect in concave mirrors of a great focal diftance, fuppofing their conftruction practicable, proceeds from the enlarged fize of the fun's image in their focus. The diameter of the image muft in all cafes be equal to the chord of an arch of 32 minutes fuppofed to be drawn from the vertex of the mirror for fuch is the angle under which the fun's disc appears to us. At great diftances therefore the diameter of the focus will be fo large, that it becomes impracticable to enlarge the furface of the mirror to fo great a degree as to produce an intense heat in it; which effect depends on the ratio of the refpective diameters of the reflecting furface and of the focus.
One great advantage of the Author's conftruction is, that the focus is variable, or may be adapted to different diftances. The moft perfect machine of this kind which he appears to have executed, confifted of 360 plain mirrors, each eight inches long, and fix in breadth, mounted on a frame eight feet high and feven feet in breadth. Every one of these mirrors was capable of being moved in all directions by means of fcrews, by the turning of which all the reflected images were made to coincide at any given diftance.-With regard to its effects-when twelve of them only were used, light combustible matters were kindled when the focus was fixed to the fmall distance of 20
feet. At the fame diftance, a large tin veffel was melted by 45 of these mirrors, and a thin piece of filver with 117. With the entire machine all the metals and metallic minerals were melted, at the distance of 25, and even 40 feet. At the diftance of 50 feet the focus, or space in which all the images coincide, is about feven inches broad; so that metals may thus be affayed, and other curious experiments be made in the large way, with the pure folar fire, which it is impoffible to execute with concave mirrors; in which the focus is inconveniently near, or weak, and generally a hundred times less than that produced by this machine.
Wood was kindled by it, when the fky was clear, at the distance of 210 feet. The diminution of power on increafing the distance of the focus, does not, as might be fuppofed, proceed from an abatement of the folar heat, in confequence of the paffage of the rays through a greater portion of air. The Author never could obferve any fenfible lofs of light arifing from this caufe, even at ftill greater diftances. The diminu tion is folely to be attributed to the neceffary enlargement of the focus as the distance increases, in confequence of the angle made by the rays proceeding from the oppofite fide of the fun's difc. On this account, at the distance of 240 feet, the focus of the combined mirrors is about two feet in diameter, that is, dilated into about four times the space which it occupied when it was at the diftance of 40 or 50 feet, and was capable of melting metals.
A machine of this kind would be useful to an experimental philofopher, who might apply it to many curious purposes. Of the feveral ufes which the Author indicates, we fhall only par ticularize the following.
We have already mentioned the affaying of metals, by means of the pure folar fire. In treating of this fubject the Author affirms that plates of pure filver, expofed to a focus formed by 224 mirrors, fent forth copious fumes, which fometimes continued to rife during eight or ten minutes before the fufion of the metal, and which were fo fenfible as to cast a shadow upon the ground. He regrets that his other occupations prevented him from executing a project which he had formed, of thus volatilising the fixed or perfect metals, gold and filver; and of condenfing and collecting the parts thus rendered volatile, by means of a proper apparatus. He recommends the profecution of this important experiment' to chemifts and philofophers, on an expectation that by thus collecting the pure vapours of the different metals, they may be more closely combined with each other, and may poffibly form compounds more intimately mixed than by fimple fufion.
This appears to us to be one of those projects which the Author ingenuously enough, and perhaps not very improperly, terms his Reveries. We think at least that we are not very fevere when we qualify with that title a propofal which the Author annexes to the preceding, and which occurred to him on reflecting upon the apparatus that might be proper to collect the metallic vapours abovementioned.
He fuppofes that by thus raising mercury into vapours, it may be frozen, while in this ftate, even in our climate, by a degree of artificial cold much inferior to that in which it was congealed, in the bulb of a thermometer, by the Ruffian philofophers at Petersburgh and in Siberia. This idea, the Author obferves, was received with approbation by fome intelligent chemifts to whom he communicated it. We cannot conceive however by what artifice mercury can be retained in a flate of vapour till it reaches the spot where it is to undergo the action of the frigorific mixture; or how the propofed artificial cold can be maintained and applied in the near neighbourhood of the heat fufficient to raise that fluid into vapour. Mercury is known to condenfe into small globules in a heat fufficient to burn the finger; and while it is in vapour, and confequently hot, it is in a very difadvantageous ftate to try the effects of a frigorific mixture upon it.-These are only a few of the many objections that may be made to the scheme of freezing mercury, by feizing the opportunity of laying hold of it for that purpose, while it is in a ftate of vapour.
The Author is of opinion that this invention may be applied with advantage in the manufacture of falt, by producing a quick evaporation of the falt-water without the expence of fuel. An affemblage of 12 mirrors, each a foot fquare, will be more than fufficient to give a boiling heat to the liquor contained in shallow pans conftructed for this purpose; a fecond or third machine being added, and placed at proper diftances, if the quantity of water is great, or extended over a large furface. He even proposes to apply this machine to the calcination of Jime-ftones, &c.
The Author terminates the fecond article with some obfervations relative to the Achromatic telescope, which are fucceeded by fome fingular propofals to form fhort telescopes of folid glafs, in order to correct the aberration caufed by the different refrangibility of the rays of light; and to construct others of very confiderable lengths, in which the whole interval or cavity between the object-glass and eye-glafs is proposed to be filled with water. We fhew our respect for the Author by abftaining from all criticism on the fubjects of thefe reveries.
The third article contains an account of the Author's numerous and laborious attempts to improve and facilitate the conftru&tion
construction of large concave burning mirrors, and convex Jenfes, intended to burn at finaller diftances. In these attempts M. de Buffon feems to have spared neither labour nor expence, nor to have been deterred from the profecution of his coftly experiments by frequent and fometimes unexpected mifcarriages, As few philofophers are in a fituation to realize fchemes of this kind, we shall give the refults of the Author's principal trials, and the views in which they were founded.
Obferving that glafs would bend to a certain degree without breaking, the Author first endeavoured to form large burning concaves by giving common mirrors, first cut into a circular form, a certain degree of concavity, by means of preffure. The force employed for this purpose was a fcrew, which paffing through a small hole made in the centre of the mirror, entered into a female fcrew in an iron bar that run across and behind the mirror, and which was fixed at its two extremities to a circular hoop of iron that served as a frame to the glass. By these means the Author fucceeded in forming burning concaves, the foci of which were variable, according to the force applied. Thofe of three feet in diameter would burn from 50 to 30 feet distance but on endeavouring to bend them fo far as to reduce the focus to the distance of 20 feet, they broke. The fame accident befel thofe of two feet in diameter; fo that at length only one of 18 inches in diameter was preferved whole, which burns at 25 feet, and which the Author keeps by him as a model of this fpecies of concaves.
Finding that these mirrors were broken in confequence of the folution of continuity made in the glass, by the hole in its centre, the Author imagined a conftruction in which this inconvenience was obviated, by employing the uniform preflure of the atmosphere in the bending of the glafs into a regular concave form. For this purpose, the circular plain mirror is to be fixed into a kind of tambour, or cylinder of iron or cop per; and part of the air contained in the cavity of this machine is to be extracted by means of a small pump adapted to it. In confequence of, and in proportion to, the degree of exhaustion, the glass mirror, will be bent by the preffure of the external air, into a concave form of a greater or fmaller radius. Such are the general outlines of the Author's fcheme, who has given figures of all the parts of this machine; but it does not clearly appear whether it was ever executed.
A more fingular and whimsical method of forming a concave by diminishing the air in the cavity of the tambour, is likewife propofed, accompanied with figures; but which, we are confident, never could be fuccefsful. The Author proposes to take off the quickfilver from a fmall circular space round the centre of a plain mirror, and to grind that part of it to the
figure of a burning lens of an inch focus. A brimstone match is to be inclosed, and fo placed, within the tambour, that, on expofing the mirror to the fun, it may be fet on fire by the lens. The Author takes for granted that the burning match will ab. forb a fufficient quantity of air, fo as to enable the preffure of the atmosphere to give the glass a proper degree of concavity.
This burning mirror, fays the Author, would be of a very fingular kind, as it would bend, and, as it were fpontaneously, become a burning glafs, merely on being exposed to the fun.' He acknowledges, however, that though it is conceived with fufficient ingenuity to deserve a place in a philofophical apparatus, it is rather curious than useful, as fome difficulty would attend the management of it.-We grant that the idea is fufficiently ingenious; but we apprehend that the whole fcheme would prove ideal in the execution, through the defection of the brimstone match, which would affuredly be incapable of effecting fuch a diminution of the air included with it, as the Author expects, or as is neceffary to convert the plain mirror into a burning concave.
M. de Buffon's fubfequent attempts to convert plain into concave mirrors, by the application of a particular degree of heat, were more fuccefsful. He conftructed furnaces for this purpofe, in which plain mirrors of ftill greater dimensions than thofe abovementioned, and fome of which were four feet eight inches in diameter, were expofed to a heat juft fufficient to foften the glass, fo as to make it conform itself to the spherical figure of the mould on which it refted, without fuffering any confiderable diminution of the polifh. Simple as this method appears, many difficulties attend the execution. The glass in particular is fubject to crack or break, notwithstanding the greatest precautions taken in the annealing of it, as well as in confequence of the fubfequent operations that may be necessary to give it a perfect figure and polish. Out of 24 mirrors treated by the Author in this manner, of which the least were above three feet in diameter, three only now remain. Two of these are 37 inches in diameter, and the other 46 inches. The laft of thefe, having been tinfoiled, was prefented to the King, and is certainly the moft powerful burning mirror in Europe.' It burns at the distance of fix feet, and the Author affirms that the heat in its focus, even after being diminished one half by its receiving the fun's rays reflected upon it from a plain mirror, in order that it may burn downwards, is ftill greater than that in the focus of Tochirnhaufen's celebrated burning lens, which is one of the most powerful that is known.
The Author tried the effects of the moon's light reflected from this powerful concave on a thermometer placed in the focus; but without finding that it produced any fenfible dila