56 



ATMOSPHK R E. 



Atmo- 

 sphere. 



Variations 

 in the pres- 

 sure of the 

 atmo- 

 sphere. 



Height of 

 the atmo- 

 sphere. 



Refraction 

 of the at- 

 mosphere. 



t above the general level of the surrounding 

 fluid, and therefore the weight of a column of air 

 reaching to the top of the atmosphere, is equal to 

 the weight of a column of water, of the same base, 

 with the altitude of 34 feet, or about 21564J pound* 

 on a square foot, or 15 pounds on every Square inch. 

 This experiment may be more easily made by using 

 quicksilver instead of water. The quicksilver will 

 rise to the height of '29 inches in the tube, and 

 will thus measure the pressure of the atmosphere. 

 Hence it follows, that the whole atmosphere ex- 

 erts the same pressure on the surface of the earth, as 

 if the surface of the globe were covered with water to 

 the deptli of 3+ feet, or with quicksilver to the depth 

 ot '29 inches. This pressure has been computed at 

 1'2,022,.56(>,000,000,000,000 pounds, or as equivalent 

 to that of a globe of lead 60 miles in diameter ; and 

 if we suppose that a man's body exposes a surface of 

 nearly 15 square feet, he will sustain a pressure of 

 S2343 pounds, or 144 tons. 



From the changes which take place in the atmo- 

 sphere, its pressure is liable to very considerable va- 

 riations. The column of quicksilver which we have 

 shown to be a measure of that pressure, varies from 

 '2S to 31 inches. The cause of these changes, which 

 are yet but imperfectly known, will be considered un- 

 der the article Meteorology. 



If the atmosphere were of uniform density, it would 

 be easy to ascertain, with the utmost accuracy, the 

 height to which it extends ; for the height of the at- 

 mosphere would obviously be to the height of the 

 mercury in the barometer, as the specific gravity of 

 common air is to the specific gravity of mercury. By 

 making the calculation on this supposition, it will be 

 found that the height of the atmosphere is a little 

 more than 5 miles. As the air, however, gradually 

 diminishes in density, the atmosphere must reach to 

 a much greater distance from the earth than 5 miles. 

 It appears from the duration of twilight, that at the 

 height of 444- miles, the atmosphere is sufficiently 

 dense to intercept the light of the sun, and reflect it 

 to the earth. We are therefore entitled to conclude, 

 that it extends to a much greater height. 



When a ray of light enters the atmosphere, it is 

 bent from its course by the same cause which re- 

 fracts the rays of light when they pass through any 

 dense medium, such as glass or water. The refrac- 

 tion sustained by light at its first entrance into the 

 atmosphere must be very small, from the extreme 

 rarity of the air. The deviation, however, will gra- 

 dually increase as it penetrates the denser strata, and 

 the ray will describe a path increasing in curvature 

 as it approaches the earth. From this property of 

 the atmosphere, the apparent altitude of the sun, 

 moon, and stars, is greater than their real elevation, 

 and they appear to be raised above the horizon when 

 they are actually below it. The refraction of the 

 atmosphere near the earth's .surface is liable to very 

 considerable anomalies. A very extraordinary' phe- 

 nomenon arising from this cause has been described 

 bv Mr Vince. The castle of Dover, concealed by the 

 hill which lies between it and Ramsgate, appeared, 

 on the 6th of August *800, as if it had been brought 

 over and placed on the side of the hill next to Rams- 

 gate. This phenomenon must have arisen from some 

 4 



variation of density in the intermediate air. Pheno- Atnuv 

 mens of the same class with the preceding have been P'* 

 illustrated experimentally by the ingenious Dr Wol- ~ 



lastou. See Edinburgh Transactions, vol. vi. p. 245. ; 

 and Phil. Trans. 1778, p. SS7 ; l 



But while the solar raj the earth's at- Modifica- 



mosphere, they suffer another change from the re- [j on * uf 

 sisting medium which they encounter. When the j?l b3r 

 suu, or any of the heavenly bodies, are considerably spuerf _ 

 elevated above the horizon, their light is transmit- 

 ted to the earth without any perceptible chan. 

 but when these bodies are near the horizon, their 

 light must pass through a long tract of air, and 

 is considerably modified before it reaches the eye 

 of the observer. The momentum of the red, or 

 greatest refrangible rays, being greater than the mo- 

 mentum of the violet, or least rel Tangible rays, t he- 

 former will force their way through the resisting 

 medium, while the latter will be either reflected or 

 absorbed. A white beam of light, therefore, will 

 be deprived ot a portion of its blue rays by its hori- 

 zontal passage through the atmosphere, and the re- 

 sulting colour will be either orange or red, accord- 

 ing to the quantity of the least refrangible rays that 

 have been stopt in their course. Hence the rich and 

 brilliant hue with which nature is gilded by the set- 

 ting sun ; hence the glowing red which tinges the 

 morning and evening clouds ; and hence the sober 

 purple oi twilight which they assume when tl 

 ruddy glare is tempered by the reflected azure of the 

 skv. 



We have already seen, that the red rays penetrate Reflective 

 through the atmosphere, while the blue rays, less power of 

 able to surmount the resistance which they meet, are the atmo- 

 reflected or absorbed in their passage. It is to this s P" erc - 

 cause that we must ascribe the colour of the skv, 

 and the bright azure which tinges the mountains of 

 the distant landscape. As we ascend in the atmo- 

 sphere, the deepness of the blue tinge gradually dies 

 away ; and to the aeronaut who has soared above the 

 denser strata, or to the traveller who has ascended 

 the Alps or the Andes, the sky appears of a deep 

 black, while the blue rays find a ready passage 

 through the attenuated strata of the atmosphere. It 

 is owing to the same cause that the diver, at the 

 bottom of the sea, is surrounded with the red light 

 which has pierced through the superincumbent fluid, 

 and that the blue rays are reflected from the surface 

 of the ocean. Were it not for the reflecting power 

 of the air, and of the clouds which float in the lower. 

 regions of the atmosphere, we should be involved in 

 total darkness by the setting of the sun, and by eve- 

 ry cloud that passes over his disc. It is to the mul- 

 tiplied reflections which the light of the sun sutlers in 

 the atmosphere, that we are indebted for the light of 

 day, when the earth is enveloped with impenetrable 

 clouds. From the same cause sober hue of 



the morning and evening twilight, which increases as 

 we recede from the equator*, till it blesses with per- 

 petual day the inhabitants of the polar regions. 



If the earth were at rest, and not influenced by 



Figure of 



any other body of the system, its own figure, and t] , ;1 , 1( ,_ 

 that of its atmosphere, would be exactly spherical, sphere. 

 In consequence of the diurnal motion ot the earth, 

 however, the figure of its atmosphere must be sphe- 



