104 TERRESTRIAL ADAPTATIONS. 



heat. They thus raise the temperature of the 

 upper regions of air, and occasion changes in the 

 pressure and motion of its currents. The clouds, 

 again, by shading the surface of the earth from 

 the sun, diminish the evaporation by which their 

 own substance is supplied, and the heating effects 

 by which currents are caused. Even the mere 

 mechanical effects of the currents of fluid on the 

 distribution of its own pressure, and the dynami- 

 cal conditions of its motion, are in a high degree 

 abstruse in their principles and complex in their 

 results. It need not be wondered, therefore, if 

 the study of this subject is very difficult and 

 entangled, and our knowledge, after all, very 

 imperfect. 



In the midst of all this apparent confusion, 

 however, we can see much that we can under- 

 stand. And, among other things, we may notice 

 some of the consequences of the difference of the 

 laws of temperature followed by steam and by 

 air in going upwards. One important result is 

 that the atmosphere is much drier, near the sur- 

 face, than it would have been if the laws of den- 

 sity and temperature had been the same for both 

 gases. If this had been so, the air would always 

 have been saturated with vapour. It would have 

 contained as much as the existing temperature 

 could support, and the slightest cooling of any 

 object would have covered it with a watery film 

 like dew. As it is, the air contains much less 



