102 The Realm of Nature CHAP. 



he sees the Sun's image at S', considerably higher in the 

 sky than it really is. In astronomical observations it is 

 necessary to correct this error, and tables of refraction at 

 every altitude of a star and for different temperatures of the 

 air have been compiled. The atmosphere, by raising the 

 apparent position of the Sun, thus serves to lengthen the 

 period of sunlight by about four minutes on the equator, and 

 by several hours and even days in high latitudes ( 124). 

 For the same reason the midnight sun is visible in places 

 where it would not appear above the horizon if there 

 were no atmosphere. Thus at Archangel in lat. 64 32', 

 nearly 2 south of the Arctic Circle, there is perpetual sun- 

 light for several days at midsummer. When from unequal 

 heating or other causes the distribution of density in the 

 atmosphere becomes irregular, light is reflected and refracted 

 by the layers of air in such a way as to make objects at a 

 great distance visible as if near at hand. This effect, which 

 is most marked in deserts and at sea, is called mirage. 

 All our knowledge of the outer regions is obtained by 

 looking through the window pane of air which encloses the 

 world, and allowance must always be made for its im- 

 perfections. 



151. Composition of Air. The experiments of Priestley, 

 Black, and Rutherford at the close of the eighteenth century 

 proved that common air is a mixture of several different 

 airs or gases, and at that date it ceased to be considered 

 an element. Innumerable analyses of air have since been 

 made which show that in all parts of the world the atmo- 

 sphere has almost the same composition. Traces of nearly 

 every gas which exists naturally, or is produced artificially 

 in large quantities, have been found in air, but the main 

 constituents are few. A rough analysis of air may be 

 made thus : (a) A large tightly -corked flask of warm 

 air when chilled by being covered with snow or 

 ice is seen to become dewed with liquid drops on the 

 inside. These drops are water, and their appearance 

 proves that water -vapour is a constituent of air. When a 

 person wearing spectacles steps from the frosty night into 

 a warm room he is the victim of an irritating variation of 



