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THE formation of HALOS has been accounted for by Mr. HUYGENS, by fuppofing, that the atmosphere may contain floating in it, certain globules, whose external coat is formed of transparent ice or water, while its central parts are opake, being formed of particles of fnow: these snowy particles being the center, around which the adjoining vapours adhere. It follows, that no effects of refracted light, are to be expected from rays attempting to pass through the center; but from thofe paffing through the fides. No. 1, a 1 2 b d c, reprefents fuch a globule; L L rays of light impinging upon it. L1, L 2, are rays, which striking the globule in 1 and 2, are confequently refracted, as entering a denfer medium, and iffue at c and d; where being again refracted, they crofs each other in e; whofe distance from the globule is fomewhat lefs than half its diameter; these rays being prolonged to 3 and 4, it follows, that no light can reach an eye placed any where in the angle 3 e 4, be these lines as long as they may. We observe, that c and d is the furtheft diftance from i, at which any ray can pafs; for the rays L a, L b, are turned much further out of their original courfes, and come out fpread much beyond 3 and 4, confequently, cannot reach the eye placed within 3 and 4.
No. 2, Represents the effect of a combination of such globules. Rays from the center drop, as appears by the
figure, cannot reach the eye at I, nor can any within the angles I 1, and I 2, (correfpondent to 3, 4, in No. 1.) for the rays from A (a a) are wide of the mark; as are those (bb) from B. But the rays from C, and D, being without 1, 2, reach I at c, and d; and being those particles which have leaft felt the refractive influence of the medium through which they have paffed, are confequently
It is obvious, that the angles 1 I 2, are a cone, within which no light is refracted to the eye; but around whose circumference effects of refraction are to be expected.
If a magnet be laid under a piece of fine paper, upon which fine fteel filings are ftrewed very thin, by means of a very gentle ftriking continued on the paper, the particles of steel will arrange themselves on the paper in forms correfponding to thofe of this figure. 1, 2, Are the poles of the magnet; 3, 4, what may be termed its equator. When direct with the magnetical axis 1, 2, the fteel filings are ftrait, as E and F, but on each fide, from 1 to I and G, and from 2 to K and H, they bend confiderably; from I and K to 3, and from G and H to 4, they gradually form themselves into yet more crooked curves: whence it is concluded, that the natural courfe of the magnetic fluid is along the polar axis, FE; and very weak towards 3 and 4. Thefe principles applied to the globe of the earth, account very readily for the polar direction of the magnetized needle. To account for the variation of the needle, we muft fuppofe a correfpondent variation of the magnetic poles of the globe. End of the FIRST Series of Lectures.
SURVEYS OF NATURE.
SECOND SERIES OF LECTURES.
LADIES AND GENTLEMEN,
HE knowledge of what preceded us, and of what paffes remote from us, is among those acquifitions which place the talents and industry of mankind in the most confpicuous point of view; and when it has for its object occurrences of great magnitude and general importance, when it traces the more extended operations of nature, and by comparison of the past and the present, in a manner anticipates the future, we cannot but regard it as among our most valuable and interesting poffeffions. With this fentiment we come now to the confideration of our terreftial dwelling; on some of whose peculiarities as a planet we have already be
ftowed our attention; obferving, its inclination, its rotation, and the variation of feafons upon it: thefe motions it poffeffes in common with other planets, but we are prevented by distance from intimate acquaintance with them, while by refidence we are naturally qualified for more accurate knowledge and clofer infpection of the earth.
We have already divided the earth into three zones, belts, or divifions, whofe natures were well expreffed by their appellations; as TORRID, TEMPERATE, and FROZEN; these will be of great use to us in the progrefs of what remains to be delivered, and will generally regulate our obfervations.
The first particular which would strike a spectator in furveying the earth, is, its unevenness, its mountains, and its valleys (these we distinguish in the moon, though little elfe is fubmitted to our fight); next its diftinctions of land and water, including the verdure of the former, and its interfections by the latter. On the first of these we propose to offer a few remarks in this difcourfe.
We begin by fuggefting, that the equatorial, or middle part of the earth, is remarkable for the dimenfions of its eminences (mountains), for the depths of its ocean, and the magnitude of its interfections (rivers); that here, mountains are highest above the level of the fea, the sea highest above the level of other feas, its bottom much below the bottom of other feas, and its rivers most capacious. Here are what we may term extremes; flat and extenfive defarts, or high and elevated mountains; few, if any, gentle declivities, or
eafy afcents; not hills, but maffes rifing to the fkies. Here are rather torrents than rivers; no gentle ftreams, no brooks fecretly gliding, but either parched waftes, or rapid, and overwhelming floods. In fupport of thefe remarks, I fhall just hint at fome of the most extraordinary of both kinds.
As a fpecimen of what mountains may be, take the accounts given us of the ANDES in South America; the most mountainous of mountains! extending from 10 deg. north latitude, to 55 fouth latitude; confequently, being in part of their course properly equatorial.
"Thefe mountains," fays OVALLE, "are a prodigy of nature, and without parallel in the world, being a high chain of hills 1500 leagues in length, and 40 leagues broad, with many intermediate valleys: the afcent is fo prodigious that we employ three or four days in arriving at the top of them, and as many more in the descent, that is, fpeaking properly, and only of the mountain; for otherwise it may be affirmed, that one begins to mount even from the fea-fide, because all the way, which is about 40 leagues, is nothing but an extended shelving coaft, for which reafon their rivers run with fuch force, that their streams are like mill-ftreams, efpecially near their fources.
"When we come to afcend the highest part of the mountains, we feel an air fo piercing and fubtil, that it is with much difficulty we breathe, which obliges us to fetch our breath quick and strong, and to open our mouths wider than ordinary, applying to them likewife our handkerchiefs to condenfe our breath, and break the extreme coldness of the air,