The antiquity of fpecula, or metal reflectors, muft, according to Plutarch, have been very great. He tells us in his life of Numa,* that it was one of the inftitutions of that prince, that if the facred fire of the vestal virgins should, at any time, be extinguifhed, that it fhould be rekindled by means of the fun's rays, collected by a polished, concave metalline fpeculum.

Aulus Gellius † quotes fome verfes of Laberius, a contemporary of Julius Cæfar, which mention a metalline burning fpeculum being conftructed. by Democritus of Abdera, a contemporary of Hippocrates, the celebrated phyfician who lived about two hundred and fifty years after Numa.‡ Reflecting fpecula were common in the time of Plautus, as appears from feveral paffages, and were then, it seems, moftly made of filver, which, however, was much alloyed with copper, § from its giving a fimell to the hands of those who rubbed it.

Vita Numæ. Numa is thought to have lived feven hundred and eight years before Chrift.

+ L. X. C. 19.

Archimedes, who lived above two hundred years after Democritus, was perfectly acquainted with burning fpecula, as appears from a work of his now remaining, wherein he demonftrates, that the parabolic form is the beft adapted to that purpofe.

Moft. A&t. I. Sc. 3. papim.

Epidic. A&t. III. Sc. 3.

Mot. A&t. I. Sc. 3.

§ Ut fpeculum tenuifti, metuo ne oleant argentum manus.

Vitruvius* appears to have been well acquainted with the proper conftruction of fpecula, as he obferves, it was neceffary they fhould be of a confiderable thickness, elfe they were apt to warp, and to reflect indiftinct images of objects. Seneca† was more completely informed on this fubject. He knew the powers of reflecting concave fpecula in magnifying objects, and fpeaks of fome other kinds that diminished, and exhibited other various diftortions of the human figure. He alfo knew, that a portion of a hollow fphere was the proper figure for the magnifying fpecula. He was alfo acquainted with multiplying fpecula, which he particularly mentions.

Pliny is ftill more particular in his account. He fpeaks of their compofition, as being of tin, and copper, which is the fame with that generally used at prefent. He fays, however, that filver fpecula were preferred, and, were firft introduced by Praxiteles, in the time of Pompey the Great. I fuppofe, he here means pure filver; for that filver was at leaft, part of the compo

* L. VII. C. 3.

+ Sunt ifta a quibusdam ita compofita, ut poffint detor quere in parvum; nam, ut dixi, funt fpecula quæ faciem profpicientium obliquent, funt quæ in infinitum augeant, & humanum habitum excedunt, modumque noftrorum corporum. Senec. Nat. Queft. L. I. C. 15.

Sectæ pilæ pars. Sen. Nat. Queft. L. I. C. 4.

fition of them in early times, appears from the paffage of Plautus above quoted. Probably, as a white metal, it might be used with the fame intent tin is at prefent, to whiten and harden the copper. Silver fpecula were however fo frequent, Pliny fays, as to be in common use with the maid fervants. He mentions the proportion of the tin to the copper, to be two of the former to one of the latter, which feems to have been that most esteemed: other proportions were equal parts of copper, lead, and tin, and another of two parts of copper, two of lead, and one of tin, but these were held much inferior, as the lead debafed the quality of the compofition very much. He mentions† various forms of them in use, as concave, convex, multiplying, diftorting, &c. Their burning quality, when opposed to the fun's rays, was likewise known to Pliny.

&c.‡

Aulus Gellius mentions feveral properties of fpecula, which fhew the nature and construction of them to be well understood in his time, fuch as the non-inverfion of objects, the appearance of an object in the centre of a concave fpeculum, and feveral others.

* L. XXXIII. C. 9. † L. XXXV. C. 17.

± L. XI. C. 107.

On

On the DIFFERENT QUANTITIES of RAIN which FALL, at DIFFERENT HEIGHTS, over the fame SPOT of GROUND, with a LETTER from BENJAMIN FRANKLIN, LL. D. By THOMAS PERCIVAL, M. D. &c. Read January 21, 1784.

T is a reflection which may mortify pride

IT

and humble arrogance, but ought certainly to animate the fpirit of patient attention, and confole us under the difappointments of philofophical purfuits, that many of the moft interefting laws of nature have remained undifcovered, till fome happy coincidence of circumftances hath pointed them out to inquiry or obfervation. Thus the energy of fire muft have been known and felt from the creation of the world; but the regularity of the expanfile power, on different bodies, is a modern difcovery, of uncertain date. And the real nature of this fubtile element, which pervades and actuates all matter, and is continually perceptible to our fenfes, is yet but imperfectly explored. The ancients were acquainted with the magnifying power of denke mediums; and Seneca has noticed, that fmall letters appear larger and brighter when viewed through a glafs globe filled with water. He has remarked, alfo, that apples are more beautiful,

beautiful, when fwimming in fuch a veffel. But thefe obfervations, which must have been made by numberlefs fpectators, in a long fucceffion of years, were regarded as folitary facts; and it was not till the thirteenth century, that spectacles were conftructed, in confequence, probably, of the experiments made by the Arabian philofopher Alhazen, and our justly celebrated countryman Roger Bacon. Yet though magnifying glaffes came then into general use, and must have been daily handled by artifts and others, three hundred years elapfed before it occurred to any one to put them together, fo as to form a telescope. The collection of watery vapours in the air, the figures of clouds, and the defcent of rain, could pafs in no age unnoticed by mankind, and have long been the fubjects of attentive investigation. Yet it is a very recent difcovery, which we owe to the fagacity of a moft ingenious phyfician and philofopher, that a manifeft difference fubfifts in the quantity of rain which falls, at different heights, over the fame fpot of ground.

A comparison having been made between the rain which fell in two places, in London, about a mile diftant, it was found that the quantity in one of them conftantly exceeded that in the other, not only every month, but almoft every time it rained. The apparatus ufed was very exact; and this unexpected variation did not

appear