76 THE POPULAR SCIENCE MONTHLY. 



the innumerable threadlets [of water], divided and subdivided like 

 the fibers of a root." 



In conclusion, it is gratifying to be able to state that all opposi- 

 tion to the reboisement law is in France a thing of the past. The 

 credits voted for the first ten years were at the rate of only two hun- 

 dred thousand francs a year. Now nearly twenty times as much is 

 readily obtained by the forest administration. The total annual out- 

 lay upon the state forests is about twelve million francs, but the 

 direct revenue derived from them is more than three times that sum, 

 besides the vastly greater incidental advantage of building up the 

 agriculture, commerce, manufactures, health, and general prosperity 

 of the restored regions, to say nothing of the diminished expenditure 

 needed to replace roads, bridges, and other structures formerly de- 

 stroyed by the torrents. How much longer shall we refuse to heed 

 what the experience of other countries teaches with regard to the 

 treatment of forests ? 



In the recent Ohio floods the States which suffered most — Ohio, 

 Indiana, and Illinois — were not those where most of the deforesting 

 was done which caused the floods. The Hudson's head-waters are 

 almost all in New York, and it is in the power of her Legislature 

 to provide the needed safeguards. 



-***- 



WHAT IS ELECTKICITYP 



Bt JOHN TROWBRIDGE, 



PKOFESSOK OF PHTSICS, HARVAED UNIVEESITT. 



THE conjunction of the meeting of the American Association with 

 the opening of the Electrical Exposition and the sittings of the 

 National Electrical Congress leads me to say a few words upon a 

 question which we all ask ourselves, and to which we have hitherto 

 had no response : " What is electricity ? " 



After I have concluded, you will probably still ask yourselves, 

 " What is electricity ? " All I can hope to do is to make you ask 

 yourselves the question with more humility, and a greater conscious- 

 ness of ignorance ; for the ignorant man, I have found, is generally 

 sure that he knows what electricity is ; and, the more learned a person 

 is, the more he is convinced that he does not know what electricity is. 



There is an advantage in sounding the depths of our ignorance, 

 and in surveying, even from a small Mount Pisgah, the paths we have 

 traversed, and the great promised land which lies before us. In the 

 beginning I must express my conviction that we shall never know 



* Address before Section B, of the American Association for the Advancement of 

 Science, Philadelphia, September 4, 1884. 



