meter, or within an evacuated glass ball, but would also electrify the glass on the outside; I immerged a piece of dry glass in a basin of mercury, and found that by taking it out the mercury was electrified minus, and the glass electrified plus to a considerable degree. I found also that amber, sealing-wax, and island crystal, when taken out of mercury, were all electrified positively.* How does it then appear that the electricty which was observed on rubbing the last mentioned substance after it was taken out of mercury surrounded by ice, was owing to cold, and not to the friction between it and the mercury in taking it out? Island crystal when warm is a non-conductor, and all non-conductors may be excited with proper rubbers. LXXII. An Attempt to assign the Cause, why the Sun and Moon appear to the naked Eye larger when they are near the Horizon. By Mr. Samuel Dunn. p. 462. The sun and the moon when they are in or near the horizon, appear to the naked eye, so very large in comparison with their apparent magnitude, when they are in the zenith, or somewhat elevated, that several learned men have been led to inquire into the cause of this phenomenon, and after endeavouring to find certain reasons founded on the principles of physics, they have at last pronounced this phenomenon as a mere optical illusion. The principal dissertations conducive to give any information on this subject, or helping to throw any light on the same, have been those printed in the Transactions of the Royal Society, the Academy of Sciences at Paris, the German Acts, and Dr. Smith's Optics; but as all these accounts had not given Mr. D. satisfaction, curiosity induced him to inquire after the cause of this singular phenomenon in a manner somewhat different from what others had done before, and by such experiments and observations as have appeared pertinent. From the common appearance of the sun near the horizon, and other like. circumstances, Mr. D. first began to suspect that a sudden dip of the sun into the horizontal vapours might some how be the cause of a sudden apparent change of magnitude, though the horizontal vapours had been disallowed to be able to produce any other than a refraction in a vertical direction; and reducing things to calculation, he found that, from the time when the sun is within a diameter or two of the horizon, to the time when he is a semi-diameter below the horizon, the sun's rays become passable through such a length of medium, reckoning in the direction of the rays, that the total quantity of medium, (reckoning both depth * A small quantity of an amalgama, or mixture of mercury and tin, with a very little chalk or whiting, being rubbed on the cushion of a globe, or on the oiled silk-rubber of a tube, will excite the. globe or tube to a great degree, with very little friction; especially if the rubbers be made more damp, or dry, as occasion may require.-Orig. 612 eye. and density) through which the rays then pass, being compared with the like total depth and density through which they pass at several elevations, it was proportionable to the difference of apparent magnitude, as appearing to the naked This circumstance of sudden increase and decrease of apparent magnitude, and as sudden decrease and increase of light, (for they both go together) seemed no improbable cause of the phenomenon, though he could not then perceive how such vapours might contribute toward enlarging the diameter of the sun apparently in a horizontal direction. eye, and He therefore examined the sun's disk again and again, by the naked by telescopes, at different altitudes, and, among several circumstances, found the solar maculæ appear larger and plainer to the naked eye, and through a telescope, the sun being near the horizon, than they had appeared the same days when the sun was on the meridian, and to appearance more strongly defined, yet obscured. Before sun-rising, when the sun has been near the tropic, and the sky at the ut most extent of the horizon has appeared very clear, and when certain fogs have appeared in strata placed alternately between the hills, and over intervening rivers, valleys, &c. so as to admit a sight of the rising sun over those fogs, Mr. D. had often observed, with admiration, the most distant trees and bushments, which at other times have appeared small to the naked eye, but while the sun has been passing along a little beneath the horizon, obliquely under them, just before sunrising, when the sun has been thus approaching towards and beneath any trees and bushments, they have grown apparently very large to the naked eye, and also through the telescope; and they have lost that apparent largeness as the sun has been passed by them. Thus a few trees standing together on rising ground, at the distance of a few miles, have appeared to grow up into an apparent mountain. Such apparent mountains formed from trees, put on all forms and shapes, as sloping, perpendicular, overleaning; but soon recover their natural appearance, when the sun is passed by them, or got above the horizon. Mountains themselves at a distance sometimes appear larger than at other times. Beasts and cattle in the midst of, and being surrounded with water, appear nearer to us than when no water surrounds them. Cattle, houses, trees, all objects on the summit of a hill, when seen though a fog, and at a proper distance, appear enlarged. All bodies admit of larger apparent magnitudes, when seen through some mediums, than others. But more particularly, He took a cylindrical glass vessel about 2 feet high, and having graduated its sides to inches, he placed it upright on a table, with a piece of paper under the bottom of the glass, on which paper were drawn parallel right lines, at a proper distance from each other; and having placed a shilling at the bottom of the ves sel, it was nearly as low as the paper. Pouring water into the vessel, and viewing the shilling through the medium of water, with one eye, while he beheld with the other eye, where the edges of the shilling were projected on the paper, and its parallels, he found the shilling appear larger, at every additional inch depth of the water; and this was the case if either eye was used; and the same when the eye was removed far from the surface, or near to it, or in any position to it. He took large vessels, filled them with water, and placed different bodies at the bottom of those vessels. It always followed that the greater depth of the water he looked through, in the direction from his eye, to the objects in the water, the nearer those objects appeared to him. Thus light bodies appeared more mellow and faint, and dark bodies rather better defined, than out of the water, when they were not deeply immersed. And thus they appeared, under whatever directions or positions he viewed the bodies. He placed different bodies in proper vessels of fair water, and immersed his face in the water; viewing the bodies in and through the water, they all appeared plain, when not too far from the eye, and though a little hazy at the edges. they appeared much enlarged, and always larger through a greater depth of water. Thus a shilling appeared nearly as large as a half crown, within a red glowing arch on that side opposite to the sun, when the sun shone on the water. From this experiment he concluded, that divers see light objects not only larger, but very distinctly in the water. These and several other circumstances being considered, they left him with but little doubt, whether the atmosphere refracts horizontally or not, as some protuberances observed in the sun's limb must have been wholly owing to such a cause, and the nearly allied strata in the atmosphere. That the apparently formed mountains of trees and bushments at sun-rising, so easily comparable with other trees and bushments of equal magnitude at other times, but in their affected state as much larger, must also be owing to the same cause. He therefore concluded that these were proofs that objects seen through a medium of greater depth or density, do appear more large; aud that therefore not only the sun and moon, but that all other objects seen at great distances under a horizontal direction, do appear larger to the naked eye, than objects of equal magnitude and distance ap pear when seen under a vertical direction. Though the quantity of medium, with its density, be here mentioned, as if it was the efficient cause of this effect, possibly it may be some other cause in the. horizontal vapours, water, and other mediums which produce effects nearly proportionate to the difference arising from a comparison of the quantity of medium. or density. Whether this effect arises from density or rarity, reflection, refraction, or inflection, acceleration, retardation, or absorbency of the rays, seems to deserve a proper inquiry. What others may find to be the cause of this phe . nomena, he cannot determine; to him it has seemed most natural that the rays under the foregoing circumstances, first become obstructed, and many of them wholly absorbed, the rest proceeding with a retarded motion, are thereby first more reflected, and then less refracted through the humours of the eye; and lastly, the image on the retina becomes hereby enlarged. In other words, certain accidents making the rays more divergent than they otherwise would be, at their entrance into the eye, seem to Mr. D. to be the cause of these and other like appearances. LXXIII. Extract of a Letter from Mr. John Bartram, of Philadelphia, to Benjamin Franklin, LL. D., F. R. S., relating to a Remarkable Aurora Borealis. Dated Philadelphia, Nov. 12, 1757. p. 474. Here is a visible aurora borealis; at 7 o'clock it was about 2 hours high, to the northward pretty bright. Soon after daylight disappeared it was much more east, where it was redder, with some faint streamers, whose points reached near 45 degrees elevation, which soon disappeared, and the light descended by degrees under the pole, and by 10 o'clock was nearly extinct. Extract of the Answer to the above Letter. Dated London, Jan. 11, 1758. By the Rev. Richard p. 475. LXXIV. Observations on Noxious Animals in England. The author relates 2 cases, both of which happened under his own inspection, and which were productive of no other mischief than that of momentary alarm. LXXV. On the Extraordinary Agitation of the Sea* at Barbadoes, March 31, 1761; and an Epidemical Disorder in that Island. By Mr. A. Mason. p. 477. There was here a very extraordinary motion of the sea, March 31, not unlike that remarked here on the dreadful day of calamity which happened at Lisbon; with this difference, the last was not so sudden as the former in the flux and reflux: which sufficiently shows, that the shock must have been greater that occasioned it, as most likely they proceeded from the same cause, viz. that of an * See another account of this agitation at p. 601, of this volume. earthquake, at some place under the surface of the sea. The tide ebbed and flowed, in about 8 minutes, between 18 inches and 2 feet, and continued so for 3 hours, regularly decreasing till night, when it was no more observable. It is very remarkable, that since that time the island had been in a very deplorable condition, having suffered under the severest colds that had been ever known. The distress had been so general, that of the inhabitants of the island had felt the effects of the contagion; and to some it had been repeated several times. Few however had died of it. The leeward islands had not escaped it, having raged there more violently and more fatal.. His majesty's ships had severely felt the effects of it, some of them not being capable of keeping the seas, for want of men fit for service. This happening at a season of the year remarkably the healthiest, made it the more surprizing. Bleeding had been found of great efficacy, and used, both by way of prevention and cure, with great success. LXXVI. Observations on Aurora Boreales in Sweden. By Mr. Torbern Bergman, of Upsal. p. 479. These are observations on the ordinary appearances of aurora boreales, and not of importance to be reprinted. LXXVII. Of the Double Refractions in Crystals. By Father John Beccaria, Professor of Experimental Philosophy at Turin. p. 486. This is now a very common and well known property in certain crystals. LXXVIII. A Catalogue of the Fifty Plants from Chelsea Garden, presented to. the Royal Society by the Company of Apothecaries, for the Year 1761, pursuant to the Direction of Sir Hans Sloane, Baronet. By John Wilmer, M. D. p. 491. This is the 40th presentation of this kind, completing to the number of 2000 different plants. LXXIX. An Account of a Work, intitled, Jacobi Christiani Schaeffer Icones et * Mr. Hudson was by profession an apothecary in the metropolis, and on the institution of the British Museum was made one of the assistant librarians in the year 1756. He resigned this office however in 1758 in order to pursue his profession. In 1762 he published the first edition of his well known work the Flora Anglica, in which the indigenous plants of England were arranged according |