98 TERRESTRIAL ADAPTATIONS. 



steam would deprive those creatures of most of 

 the other external conditions of their well-being. 



The real state of things which we enjoy, the 

 steam being mixed in our breath and in our sky 

 in a moderate quantity, gives rise to results very 

 different from those which have been described. 

 The machinery by which these results are pro- 

 duced is not a little curious. It is in fact, the 

 machinery of the weather, and therefore the 

 reader will not be surprised to find it both com- 

 plex and apparently uncertain in its working. 

 At the same time some of the general principles 

 which govern it seem now to be pretty well made 

 out, and they offer no small evidence of benefi- 

 cent arrangement. 



Besides our atmosphere of aqueous vapour, 

 we have another and far larger atmosphere of 

 common air; a permanently elastic fluid, that is, 

 one which is not condensed into a liquid form by 

 pressure or cold, such as it is exposed to in the 

 order of natural events. The pressure of the dry 

 air is about 29* inches of mercury ; that of the 

 watery vapour, perhaps, half an inch. Now if 

 we had the earth quite dry, and covered with an 

 atmosphere of dry air, we can trace in a great 

 measure what would be the results, supposing 

 still the equatorial zone to be hot, and the tem- 

 perature of the surface to decrease perpetually 

 as we advance into higher latitudes. The air at 

 the equator would be rarefied by the heat, and 



