FOREST INFLUENCES. 11 



apparent. Wlieu we find our means of measurement so deficient we 

 must be careful how far we base conclusions on their records. 



In this connection the very suggestive paper by Mr. George E. 

 Curtiss, contained in this bulletin, shonld receive attention. His clas- 

 sification of rains into convective, orographic, and cyclonic will cer 

 tainly assist in developing- true conceptions as to cause and effect and 

 possible relation of surface conditions to rainfall. 



While, then, conclusive deductions may perhaps not be as yet ad- 

 missible, we can not refrain from pointing out the results obtained in 

 the forest station at Lintzel (recorded on page 113), which seem to 

 show that forest planting did, under the conditions there prevailing, 

 produce a considerable change in meteorological conditions. 



We can understand readily that if any influence exists it must be 

 due, in the first place, to the mechanical obstruction which the forest 

 cover presents to the passage of air currents and to the action of the 

 sun's rays upon the soil— it must result from a difterence in insolation 

 and consequent differences in temperature and evaporation over forest 

 and field. It is also readily uuderstood that the influence can become 

 appreciable only when large enough areas exhibiting such dift'erences 

 are opposed to each other, capable of producing local currents of air 

 which may intercommunicate the characteristics of the one area to the 

 other. The size and character of the forest growth, its density, 

 height, situation, and composition, are, therefore, nuich more impor- 

 tant in determining- its influeuce than has been hitherto supposed. 

 It is not trees, but masses of foliage, which may be effective. A large 

 sheet covering- an extended area from the influence of the sun would 

 produce almost the same differences in meteorological conditions that a 

 forest cover is expected to produce. 



While, then, we may admit a priori that extent or area and condi- 

 tion of the forest cover are important, we have as yet no data from 

 which to calculate any proper size or proportion, and the attempts to 

 fix a certain percentage of forest cover needed for favorable climatic 

 conditions of a country are devoid of all rational basis. 



Leaving the question of forest influences upon climate as still await- 

 ing final solution, we may speak with much more confidence of the 

 ettect which forest cover exerts upon the disposal of water supplies. 

 This effect can be much more readily studied and shows itself nuich 

 more conspicuously. It is perhaps also much more important to human 

 economy, for it is becoming more and more apparent that our agricul- 

 tural production is dependent not so much upon the amount of rainfall 

 as upon the proper disposal of the waters that fall. 



Eecognizing this truth, the American Association for the Advance- 

 ment of Science, iu 1891, sent the following resolution to the Secretary 

 of Agriculture: 



The American Association for the Advancement of Science respectfully submits for 

 the cuusideratiou of the Secretary of Agriculture that the future of successful and 

 more productive agriculture depends very largely upon a rational water manage- 



