Puslapio vaizdai
PDF
„ePub“

OBSERVATIONS ON THE PLATES

Belonging to LECTURE VIII

PLATE I.

IG. 1. represents a general idea of the benefits refulting

FIG

[ocr errors]

to the earth by the intervention of an ATMOSPHERE, This is fuppofed to be the northern hemifphere; the north pole in the centre of the figure; the Sun in the equator, confequently the Equinox.

It appears by the figure, that those parts which have lately, by the diurnal rotation of the earth, turned from the fun, and are lofing fight of him (i. e. the part marked Evening), yet enjoy his beams by the refraction of the atmosphere: were no fuch medium around it, a would be involved in night; whereas, night is but just sensible at d, and the interval between thofe letters, is gradually preparing for the total absence of the folar luminary. At Noon the atmof phere fpreads the rays of light all around, and diffuses equal and general brightness. In Morning, as the parts of the globe, b and c, turn toward the fun, the light gradually advances, fo that, although in ftrianefs only b fhould be illuminated, yet the atmosphere extends the folar rays to c, and prepares thofe parts for perfect day. Thus it ap pears, that though the line a b, should be the natural boundary of day and night, yet by this invaluable contrivance,

much

[blocks in formation]

much beyond that line is rescued from darkness, and c d is the actual boundary of light and fhade. In fact, therefore, the very pole has now no night; though all other parts of the globe have equal day and night.

It is fuppofed in this figure, that the rays of the fun fall parallel on the earth; but this we have already obferved they do not, but illuminate more than half the globe conftantly; by which fuperiority, efpecially when aided by the atmosphere, darkness is very much leffened, and light increafed, for the purposes and enjoyments of life.

This figure is a kind of companion to that in the last Lecture, explaining the rotation of the earth: the circle in which the fun is fuppofed to move, correfponds to that marked EQ, and the effect of the fun, is what would take place when he is in the ftation f 7. From the combination of these figures, seen in such different aspects, a very clear idea of the diurnal rotation and its effects may be attained.

Fig 2. Is a more diftinct view of twilight than could be given above we observe, that the ray from the fun to I, which would proceed direct to a, is by the atmosphere at I, caught and inflected to 2, just striking A in its paffage; and thereby rendering the fun vifible to A, although that orb be below his horizon, which is 1, 2. This ray is again deflected from 2 to 3, where night begins; the fun being full eighteen degrees below the horizon of any fpectator fuppofed at 3. The space from A to 3 is gradually darkened, till at 3 it becomes totally obfcure, and void of light.

PLATE

[ocr errors]

PLATE II.

IG. J. In this figure, the ATMOSPHERE is divided into feveral parts, according to the fuppofition hinted in the LECTURE. The groffer divifions next the earth, the lighter and thinner parts of the fluid further from it. If a ray from a heavenly body be fuppofed to proceed direct and ftrait from the ftar to the firft divifion, it is there a little inflected and bent downward; paffing into the fecond divifion, it is fomewhat more bent; in the third it is yet more attracted from its original courfe, till by the time it has reached A. it is confiderably incurvated from its first direction; but as A fees this ftar by its light, and its light ftrikes his eye in that direction which it has acquired in the groffer medium, he will naturally suppose the object to be in that direction prolonged, and of courfe will eftimate the ftar at a, which, in fact, is below that station. If A be fuppofed infpecting an object in the zenith Z; he will fee it in its true place; becaufe, though the rays, as before, país out of a thinner medium into a groffer, yet falling right upon every fuch medium, their courfe continues to be direct and ftrait forward, and fuffers no refraction; but reaches A in the fimpleft manner poffible, and alfo through the leaft poffible track of air, vapours, &c. If A is imagined to be looking at the object b, which is in his natural horizon, he will see it through the greatest poffible quantity of vapours, and through the greateft poffible quantity of that part replete with the groffeft vapours; the diftance from A to b being more than from A to b, and the comparative distance from A to the firft line, very much longer on the line A b, than on the line A b; consequently, here refraction is most powerful, and though never equal to that (employed to render it more evident

and

« AnkstesnisTęsti »