252 PHYSIOGRAPHY 



even though the temperature is far below that of melting. This 

 is shown by the fact that snow and ice disappear slowly even when 

 the temperature is below 32 F. A wet cloth put into a very low 

 temperature, say F., freezes stiff; but if it stays at the same tem- 

 perature long enough, it becomes dry. The ice in it has evaporated. 



All animals breathe out water vapor into the atmosphere. This 

 is seen in winter, when the water vapor of the breath condenses, 

 and so becomes visible, in the cold atmosphere. The water 

 breathed out is not seen in summer, or in a warm room, because 

 it does not condense in warm air. Plants also breathe out moist- 

 ure, the amount being very great. In some cases it has been actu- 

 ally measured. 1 Thus a thrifty sunflower plant, during its life of 

 140 days, gave off 125 pounds of water. Grass was found to give 

 off its own weight of water every 24 hours, in hot weather. This 

 would mean 6}^ tons per acre, and a little more than one ton for a 

 lot 50 feet by 150 feet. A birch-tree, with some 200,000 leaves, was 

 estimated to give off 700 to 900 pounds on a hot summer day, though 

 very much less on cool days. Much water vapor also escapes from 

 active volcanoes (p. 169). 



The oceans must be looked upon as the great reservoirs from 

 which most water comes, and but for them the waters of the land 

 would all be dried up in the course of time. It is true that the 

 oceans receive water from rivers, springs, and rains about as fast 

 as they lose it by evaporation; but the water which falls as rain is 

 largely from the ocean, and if the ocean stopped yielding the 

 water vapor which furnishes the water for the rain, all the waters 

 of the land would dry up. 



Water is in constant circulation in the air. The circuit which 

 it makes is somewhat as follows: It is (1) evaporated from the 

 ocean, then (2) diffused or blown over the land, where some of it 

 (3) falls as rain or snow, feeding rivers, springs, lakes, etc. A 

 part of this water which falls from the air returns to the sea, while 

 another part is evaporated into the air again without flowing to the 

 sea. The evaporation of water and the circulation of the water 

 vapor in the air are therefore important, not only for us, but for 

 all living things. 



1 Bergen, Foundations of Botany, p. 161. 



