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PLATE V.

DAILY ROTATION OF THE EARTH.

THIS plate exhibits the course of an equinoctial day. The earth's rotation from Weft to Eaft, in the order of the figures 1, 2, 3, 4, &c. Or, suppose the line 11. takes in an hour's time the place of that marked 12, the line 10 the place of 11, and fo on, then 1 becomes 2, 2 2 becomes 3,3 becomes 4, &c. and all in fucceffion will point at any object fituated above and beyond the rotation. Such an object therefore will, to the inhabitants of the globe, advance in a contrary direction; as a, b, c d, &c. according to the order of the letters. If we fuppofe a circular line; as that which goes round the figure, to be elevated over it on the equator E Q any fixed object in that line, would seem to advance in the order of the letters. We admit this object to be the sun. Wherever the fun is vertical it is noon. Suppofe him vertical at Q, it is forenoon to a, b, c, d, e; morning to f; night to all beyond. As f gradually turns to him, it is at fnoon, forenoon to g, h, i, k, l, day-break to m, and night to all beyond and it is afternoon to e, d, c, b, evening to a, night to all beyond. As the earth continues her rotation, the fun feems to advance to e, where it is noon to E, afternoon to m, l, k, i, b, g, evening to f, night to all beyond. The figure fhews, that when it is noon, evening, or night, to any fpot under the line E Q, it is the fame time of day to all places north or fouth of that line: As when the fun is on the mid-day (meridian) line, f 7. it is not only noon to f and to 7, but alfo to A and to B; and it is equally day-break to HE and L: and fo of all other meridians.

PLATE

PLATE VI.

Fig. 1. fhews various lengths of the fhadow from any object, as occafioned by the fun at his different ftations in the ecliptic. S his winter ftation; the fhadow prolonged on a pavement to 3: S 2 the equinox; fhadow fhortened to 2 : 3 the fummer folftice; the fhadow approaching to the perpendicular of its object at 1. a, b, c fhew the fame effect fuppofed to be noticed on a wall or other perpen

S

dicular.

Fig. 2. Shews that the fpace is much lefs between those rays of light, &c. which fall immediately perpendicular, as 1, 2, than between thofe which fall obliquely, as 3, 4 : confequently fewer rays, and thofe lefs powerful, fall on any equal fpace on the lower line than on the upper. This effect would be greater ftill at a greater obliquity, as 2, 3. Fig. 3. fhews, that if the rays of light fall nearly direct 1, 2, and are reflected in the fame angle 2, 3, from whence they are again returned 3, 4, a much greater number will fall on a given space, and with a greater effect, than when, as in fig. 4, the angle at which they fall is greater 1, 2, and when fallen they rebound farther 2, 3, and are reflected again to fo wide an angle as 3, 4.

This plate explains the reafoning introduced in the lecture

page 133.

LEC

LECTURE VIII.

F but now and then, we were favoured with opportuni

I ties of noticing fuch interefting particulars as are con

ftantly occurring around us, our curiofity might be excited, and our attention awakened; not that the facts themfelves would be more interefting, or more useful, but more rare. In human nature, the neglect of what is common is a frequent failing novelty has charms, for which we forfake conftant utility. This is the cafe with regard to our notice of benefits received from our ATMOSPHERE: I have often wondered at it, having frequently been in company among perfons of talents, who yet were little informed of the nature of that medium, on which depend all the enjoyments of fight, and perhaps of all our fenfes. But we are at this time, LADIES and GENTLEMEN, diffenting from popular inattention, and defirous of acquiring information on this important ar

ticle.

Let me first lead your thoughts to what might be our fituation, had the earth no atmosphere encircling it; what a defert! what a waste! The fun in the firmament would feem like a fire in the night, glaring and fierce, ftrongly contrasted on a back-ground of intenfe black, overpowering indeed the ftars clofe to him, and thofe only. No others would “hide "their diminished heads," but ever acccompany his daily appearance:

appearance fuch would be the fcene in the heavens! On earth, we fhould be overwhelmed with the refplendence of that diminutive portion immediately adjacent to us; while, on either hand, at but a little diftance, reigned obfcurity and night; farewell the gentle declivities of rounding bodies, farewell reflex lights, intermingling colours, the infinite variety of compounded tints, the delighting diverfity of brilliant harmony! Behold, inftead, light, infupportably fplendid! and darknefs, the fhadow of death! Farewell the voice of mufic refounding through the grove; farewell refpondent echo, if not the fprightly vivacity of cheerful converfation; farewell gentle warmth, if not heat; or gentle coolnefs, free from freezing cold.

But as my design at prefent is rather to confider the visible effects of the atmosphere; I propofe the following thoughts for your attention.

The atmosphere reaches in extent over our heads to a very confiderable distance, perhaps two or three hundred miles or more; but at its extreme height is fo very thin, attenuated, and rare, as to lose itself infenfibly in æther. We shall therefore only affume its height from the earth, where it is found to be of any use to us, i. e. about forty-five miles; at which diftance it is fenfible by the effect of light.

The atmosphere is TRANSPARENT, whereby we are permitted to view an infinite number of fhining bodies through it, whofe light paffes into this medium, and fo to our fight: yet, compared with the adjacent æther, the atmosphere is DENSE and compact, indicating, as it were, the approach to

3

a folid

a folid body; by which denfity it attracts the rays of light passing near it, and inflects them for our use, as one may say, before they defigned it; which I explain thus: all rays, in a uniform medium, pass in strait lines (the æther is a uniform medium); but paffing from one medium into another, they are acted upon by this fecond medium, according to its nature. If it be thinner than the medium they quit, they rife upwards; if it be denfer than the medium they quit, it attracts them to itself, and they bend downwards. Such is the nature of the atmosphere; it attracts the paffing rays of light, and bends them down to earth, long before they would ftrike it, if their courfe was not thus diverted. When the fun is below the horizon more than eighteen degrees, his rays fhoot into empty space; but when at that point, the atmofphere feels the welcome vifitants, and directs their courfe, by its refractive power, down to the furface of our globe: this is the first point of twilight; which, as the folar ray's fucceed cach other, and increase in quantity, iffues at length in morning.

I should obferve, that TWILIGHT varies according to the ftate of the atmosphere; so that sometimes, when the fun is more than eighteen degrees below the horizon, his rays are inflected. For if the atmosphere be elevated by heat, it may fooner catch the folar rays; if it be depreffed by cold, it will be later ere it feels their influence. On this circumstance depends the duration of twilight; and this renders twilight longer in the evening than in the morning, and in warm weather, than in cold.

VII.

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If

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