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, fuppofe the line 11. takes in an hour's time the place of that marked 12, the line 10 the place of II, and fo on, then 1 becomes 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 feem to advance in the order of the letters. We admit this object to be the fun. 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 fgradually 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, h, 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

PLAT E 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 supposed to be noticed on a wall or other perpendicular.

S

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 those lefs powerful, fall on any equal space on the lower line than on the upper. This effect would be greater ftill at a greater obliquity, as 2, Fig. 3. fhews, that if the rays of light fall nearly direct 1, 2,

3.

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