RELATIONS OF TREES TO THE ATMOSPHERE. 141 



and let the other stand with water only. In a few hours 

 the water will disappear from the cup containing the 

 plants, while that in the other cup will not be sensibly 

 diminished. Indeed, there is reason to believe that gal- 

 lons of water might be evaporated into the air by keep- 

 ing the cup containing the cuttings always full, before 

 the single gill contained in the other cup would dis- 

 appear. If a few cuttings will evaporate a half-pint of 

 water in twelve hours, we can imagine the vast quantity 

 constantly exhaled into the atmosphere by a single tree. 

 The largest steam-boiler in use, kept constantly boiling, 

 would not probably evaporate more water than one large 

 elm in the same time. 



We may judge, from our experiment with the cuttings, 

 that a vastly greater proportion of moisture would be ex- 

 haled into the atmosphere from any given surface of ground 

 when covered with vegetation, than from the same amount 

 of uncovered surface, or even of standing water. Plants' 

 are indeed the most important existing agents of nature 

 for conveying the moisture of the earth into the air. The 

 quantity of transpiring foliage from a dense assemblage 

 of trees must be immense. The evaporation of water 

 from the vast ocean itself is probably small compared 

 with that from the land which it surrounds. And there 

 is reason to believe that the water evaporated from the 

 ocean would not produce rain enough to sustain vegeta- 

 tion, if by any accident every continent and island were 

 deprived of its trees. The whole earth would soon be- 

 come a desert. I would remark, in this place, that trees 

 are the agents by which the superfluous waters of the 

 ocean, as they are supplied by rivers emptying into it, are 

 restored to the atmosphere and thence again to the sur- 

 face of the earth. Trees pump up from great depths the 

 waters as they ooze into the soil from millions of sub- 

 terranean ducts ramifying in all directions from the bed 

 of the ocean. 



