vii The Atmosphere 101 



iol miles above the Earth's surface J, at 14 miles ^, at 17^ 

 miles ^ , and at 2 1 miles only ^ of |rfe atmosphere lies at 

 a higher level : at 2 1 miles the barometer would stand 

 at half an inch. Thus, if Boyle's law holds good the 

 atmosphere has no definite limit, but extends with diminish- 

 ing density throughout infinite space. It has however been 

 proved that this law does not hold for gases of very small 

 density, which behave like very light liquids and have a 

 definite surface, so that the atmosphere has an upper limit, 

 beyond which the particles of gas do not stray. 



149. Height of the Atmosphere. Observations of 

 twilight ( 162) show that the atmosphere is not less than 

 45 miles high. The aurora, which is produced in the 

 upper atmosphere ( 174) has been measured at more 

 than 100 miles above the Earth, and meteors ( 134) 

 sometimes become visible at 200 miles. Hence it is prob- 

 able that the atmosphere extends at least 200 miles beyond 

 the Earth's surface ; but in consequence of its compres- 

 sibility nearly three quarters of the air lies between sea-level 

 and the summit of the loftiest mountain. 



150. Atmospheric refraction. When light from any 

 of the heavenly bodies enters the atmosphere, it traverses 

 denser and denser layers, and is consequently bent down- 

 ward from a straight line 



as it approaches the sur- 

 face ( 6 1 ). The amount 

 of this bending or refrac- 

 tion is proportional to the 

 obliqueness of the rays of 

 light thus when the light 

 falls perpendicularly from 

 the zenith there is none, FlG - "--Atmospheric Refraction. A, ob- 



' server; b, true position; S. apparent 



but when it COmeS parallel position of Sun. The density of the 



to the horizon the refrac- JSSbS! *^ by the c?oseness 

 tion is great. A person 



always refers an object to the direction from which the 

 light enters the eye. When the Sun is near the horizon 

 its light is bent into the curve SA (Fig. 21) and as the light 

 reaches the eye of an observer at A from the direction S'A, 



