8i8 THE POPULAR SCIENCE MONTHLY. 



and little has been done to grapple with them. What is known 

 of these subjects is as yet almost exclusively empirical. Instru- 

 mental appliances are here far in advance of theories, and it is not 

 to be disguised that great waste of labor too frequently results 

 from an exaggerated refinement in observation, and subsequent 

 numerical computation, which has no real value. The variations 

 of the temperature, of the pressure, and of the motion of the air, 

 and of the quantity of vapor it contains, give rise to the great 

 series of phenomena which are included under the general term 

 climate. Of these variations the primary causes are the action 

 and reaction of the mechanical and chemical changes set up by 

 the sun's heat as influenced by the earth's motion, terrestrial posi- 

 tion, and the condition of its surface, as well as by fluctuations of 

 the sun's heat itself, though of these last we know too little to do 

 more than recognize their presence. 



The conditions which determine at any place the greater or 

 less degree and duration of direct exposure to solar radiation, and 

 therefore the quantity of heat received there, are position in rela- 

 tion to latitude, combined with the diurnal and annual movements 

 of the earth. The nature of the surface regulates the local accu- 

 mulation of heat, by reason of the varying power of absorption or 

 radiation possessed by different substances ; while with elevation 

 above the sea-level as the density of the air becomes less, the 

 sensible temperature and the quantity of watery vapor are sub- 

 ject to corresponding change. The whole of the results thus 

 produced, moreover, are modified by movements in the air con- 

 sequent on atmospheric changes from place to place, or from time 

 to time. 



The inequalities of the earth's surface, which are insignificant 

 when viewed in relation to the whole globe, are of the greatest im- 

 portance in relation to the atmosphere. For, owing to the laws of 

 elastic fluids, the great mass of the air and of the watery vapor it 

 contains are concentrated very near the surface. One fourth of 

 the air and one half of the vapor are found below 8,000 feet from 

 the sea-level ; one half of the air and nine tenths of the vapor are 

 below 19,000 feet, which hardly exceeds the average elevation of 

 the highest ranges of the Himalaya Mountains; while three 

 fourths of the air and virtually the whole effective vapor lie below 

 30,000 feet, and therefore within the influence of the highest sum- 

 mits of those mountains. That portion of the atmosphere which 

 is nearest the surface is manifestly the most likely to be acted 

 upon by irregularities of relief, and by local variations in the 

 power of absorbing or radiating heat or diffusing vapor. Hence 

 it is certain that it is the movements of the lower strata of the 

 atmosphere that chiefly affect all conditions of climate, though no 

 doubt there are great movements in the upper regions to bring 



