210 HISTORY OF 



very little resemblance to that jnire elementary body which was 

 described in the last chapter ; and which is rather a substance 

 that may be conceived, than experienced to exist. Air, such 

 as we find it, is one of the most compounded bodies in all nature. 



however, that certain acid fumes, as those of the muriatic acid, nitric acid, 

 and above all, of the oxymuriatic acid, have the property of destroying these 

 miasmata, or at least of preventing them from producing deleterious eflfecta 

 on the animal economy. 



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 atmosphere would obviously be to the height of the mercury in the barome- 

 ter, 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 atumsphere is a little more than 5 miles. As the air, however, gradu- 

 ally diminishes in density, the atmosphere must reach to a much greater 

 distance from the earth than 5 miles. It appears from the duration of twi. 

 light, tliat at the height of H^ 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 cour.«e by 

 the same cause which refracts the rays of light when they pass through any 

 dense medium, such as glass or water. The refraction 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 gradually increase as it pene- 

 trates the denser strata, and the ray will describe a path increasing in curva- 

 ture 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 ele- 

 vation, and they appear to bo raised above the horizon when they are actu- 

 ally below 't. The refraction of the atmosphere near the earth's surface is 

 liable to very considerable anomalies. A very extraordinary phenomenon 

 arising from this cause has been described by Mr Vince. The castle of Dover 

 concealed by the hill which lies between it and Ramsgate appeared, on the 

 6th (if August, 1806, as if it had been brought over and placed on the side of 

 the hill next to Ramsgate. This phenomenon must have arisen from some vr- 

 riation of density in the intermediate air. Phenomena of the same class with 

 the preceding have been illustrated experimentally by the ingenious Dr Wol- 

 (aston. See Edinburgh Transactions, vol. vi. p. 2-15 ; and Phil. Trans. 1778, 

 p. 357 ; 1798 But while the solar rays traverse the earth's atmosphere, they 

 suffer another change from the resisting medium which they encounter. 

 When the sun or any of the heavenly bodies are considerably elevated above 

 the horizon, their light is transmitted to the earth without any perceptible 

 change ; 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 momentum of the violet, or least refrangible rays, the 

 former will force their way through the resisting medium, while the latter 

 will be either reflected or absorbed. A white beam of light, therefore, wilJ 

 by deprived of a portion of its blue rays by its horizontal passage tlirougb 



