and distinct) from a to the star, yet it elevates objects on the horizon full half a degree

Fig. 2. If refraction be augmented, or leffened according to the ftate of the air, it is certain it will produce correfpondent effects on the horizon. A fpectator A, may be fuppofed to furvey the fweep around him 1 2 (expreffed by the dotted line), for here his vifual ray ftrikes the circumference of the globe, and this would appear level to him on the line bo: but if a body of aerial vapour be distributed around him, it refracts the rays coming to him from much beyond the circle 1 2, and he views a wider circumference; feen level on his horizon bo at a: if the air be fuppofed yet more replete with tranfparent vapours, it proportionably extends his furvey, and he now fees the circle 3 4, which on his horizon ho appears extended on a level, and terminates at b.

Suppofing the eye of the fpectator A to be five feet high, the diftance A 1 is four miles and almost two-thirds; i. e. 8125 yards.

No determinate distance can be affigned for the extent of the refracted horizon; unless the state of the air, and part of the world is known for, as already hinted in the LECTURES, the polar inhabitants see the heavenly bodies, &c. on their horizon, through a much greater proportion of air, and alfo of grofs vapours, than thofe at the equator: infomuch, that the fun regularly, at fome feafons of the year, rifes at the poles of an oval figure; his lower limb being heightened by refraction, and brought nearer to his upper, while his horizontal diameter remains unchanged. We learn from the teftimony of the Hollanders, who, A. D. 1596, wintered in Nova Zembla, in latitude 76°, that by the refraction of the atmosphere, they faw the funfeventeen days fooner, than by theoretic calculation, they ought to have done.

F'

PLATE III.

IG. 1. and 2. Refer to Mr. Hawkfbee's Experiment for afcertaining the refractive powers of the ATMOSPHERE, Fig. 1. The prifm is charged with an additional quantity of air, rendering its contents double what they would be in free air in this compreffed ftate of the air in the prifm, the object appears at A; as the compreffed air is let out, the object defcends to its true ftation in open air; i. e. from A to 1 and 2: which is a defcent of ten feet and a quarter.

Fig. 2. The air contained in the prifm being gradually drawn away, the object defcends from its true ftation in opén air (b) to a and A (ten feet and a quarter): upon the re-admiffion of air into the vacuum, the object rifes from A, to a and b; merely by the refractive powers of the air fo re-admitted.

SOUND. Fig. 3. This figure endeavours to represent the waves of agitated air, conveying found from the vibrating body A. At first they are strong as 1, 2, 3, 4, but weaken as they recede from A to B; whofe interpofition prevents their progrefs, except at the orifice B, through which they proceed, as it were, afreth, in a,b,c,d,e; weakening as they go, till they become infenfible (i. e. the found becomes inaudible). Thofe rays which are stopped by the wall B, return, and form an ECHO, which weakens correfpondently to its diftance from the reflecting body, as 1, 2, 3, 4, 5,6, and at length becomes totally quiefcent.

According to the fituation of an auditor, he will hear the ori

ginal found, and the echo more or lefs feparate; at echo 1, they will be almost union, but at echo 6, they will be

diftant.

LECTURE