40 PRINCIPLES OF AMERICAN FORESTRY. 



very great progress westward; but with man's assistance in 

 cultivation and various other ways, it may be extended 

 much farther towards the arid regions than if left to natural 

 conditions. So we find that, while great sections of the 

 interior of this country are treeless on account of lack of 

 water, trees planted on them and properly cared for may 

 often grow thriftily. But trees planted on our prairies 

 always require more care to make them do well than those 

 planted in sections of greater rainfall, and we should not 

 expect them to grow as large as in the timbered sections 

 without irrigation. 



RAINFALL AND HEIGHT OF WATER TABLE 

 IN THE LAND. 



A few years ago it was argued by many friends of tree- 

 planting that it was practicable by the planting of trees to 

 increase the rainfall and prevent evaporation in the great 

 continental plain sufficiently to materially change the cli- 

 mate. The large rainfall and the good crops produced for 

 a number of years in the drier portions of this area, after 

 considerable planting had been done, seemed to indorse all 

 that the most enthusiastic of tree-planters claimed. But it 

 must be very evident to any careful student of the subject 

 that such small plantings as were made, even had they been 

 maintained, could scarcely have had any appreciable effect 

 on the general climate of so vast a territory. It is very 

 evident, too, from a study of the annual rainfall, that it has 

 fluctuated greatly in this section, and that we have per- 

 haps not recorded the least or the greatest amount for any 

 one year. 



There are some facts that seem to show plainly that 

 there must have been a time when the water-level of our 

 lakes was much lower than it is now or than it was during 

 the very dry years from 1890 to 1895, when the rainfall in 



