102 TERRESTRIAL ADAPTATIONS. 



indications. And here again we find something 

 to remark. 



In both the simple atmospheres of which we 

 have spoken, the one of air and the one of steam, 

 the property which we have mentioned must 

 exist. In each of them, both the temperature 

 and the tension would diminish in ascending. But 

 they would diminish at very different rates. The 

 temperature, for instance, would decrease much 

 more rapidly for the same height in dry air than 

 in steam. If we begin with a temperature of 80 

 degrees at the surface, on ascending 5,000 feet 

 the steam is still 76! degrees, the air is only 64J 

 degrees ; at 10,000 feet, the steam is 73 degrees, 

 the air 48i degrees; at 15,000 feet, steam is at 

 70 degrees, air has fallen below the freezing point 

 to 31 J degrees. Hence these two atmospheres 

 cannot exist together without modifying one ano- 

 ther : one must heat or cool the other, so that 

 the coincident parts may be of the same tempera- 

 ture. This accordingly does take place, and this 

 effect influences very greatly the constitution of 

 the atmosphere. For the most part, the steam is 

 compelled to accommodate itself to the tempera- 

 ture of the air, the latter being of much the greater 

 bulk. But if the upper parts of the aqueous 

 vapour be cooled down to the temperature of the 

 air, they will not by any means exert on the lower 

 parts of the same vapour so great a pressure as 



