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PLATE III.

FIG

'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 prism, 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 open 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 original found, and the echo more or lefs feparate; at echo 1, they will be almoft union, but at echo 6, they will be diftant.

LECTURE

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LECTURE IX.

MIXTURE of good and ill, characterises the prefent

state; our benefits are alloyed with attendant evils; and grateful alleviations accompany forrows. On this terrestrial globe, life, pleasure and delight, are brothers to death, anguish and mifery. Ever various, the mantle which furrounds our habitation! it is now replete with elegance and falubrity, is the splendid robe of royal magnificence: presently, behold darknefs and gloom terrific! flaming fire devours, thunder shakes the earth, the torrent pours its rapid force, or hail defcends in dreadful volleys, or the whirlwind fpreads sweeping devastation. -Is the deluge paft? The reverfe is equally fatal:-the Heaven with-holds its rain, no gentle dew defcends, no cooling fhowers; "the heavens above are iron, the earth beneath is brafs," "the cattle forfake the field where they drop their young; the hufbandman covers his head, defponding for his tillage; the nobles fend their little ones to the waters, they come to the pits; but return with empty veffels, afhamed, confounded, and covering their heads." Or, inftead of health, the atmosphere diffufes mortality; flight is vain, the contagion overspreads a country! remedy is vain, we breathe the peftilence!-No longer a fplendid robe; our mantle is now the en

VIII.

M m

venomed

venomed garment which confumes the vitals of the wearer. Happy Britain, which only by report conceives of these calamities, and only by sympathy suffers these miseries! To us fuch defolation is unknown :-yet we fhould never forget that period when London was verdant with grafs, for want of paffengers to tread the streets.

But at this time, LADIES and GENTLEMEN, the atmosphere which furrounds us is to be confidered with regard to thofe operations which are conftantly carrying on in it, and which more immediately strike our obfervation as belonging to the subject. First, we treat of WINDS; and these we fhall divide into CONSTANT, and INCONSTANT.

When I attribute conftancy to the wind, I am aware that I seem to contradict daily experience; to this I reply, we must not fo depend even on experience itself, as to exclude facts which oppose it for though among us the vane is no emblem of steadiness, till "it ruft to a point, and fix at laft," yet, in the proper latitudes of the Pacific ocean, the wind blows steady, in one uniform direction, and with one continued velocity; the feaman fets his fails, and almost forfakes the helm; night and day the breeze fwells his canvafs, and wafts his veffel in full security; the wide-extended waters fear not tempeft or storm; never deformed by contending billows. Such is the description of fome parts of the great fouth fea.

There is something striking, to this purpofe, in Commodore ANSON'S Voyage in the fouth fea. We read, at one part, of raging waves beating the fcarce floating fhip, raifing

her

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