SECRETARY'S REPORT. 61 



sufficient vapor in the air to support them. If, just after this 

 local rain, a general storm begins, or one which might, under 

 other circumstances, have been general, it moves on till it ar- 

 rives at the place of the former rain. But this previous rain 

 has already exhausted the surplus of moisture which the air 

 at that place contained, and the second storm must come to an 

 end; whereas, if there had been no local rain in this first place, 

 there would have been sufficient moisture to enable the general 

 storm to continue on its course. On the other hand, if a gen- 

 eral rain occurs, extending over a large space of country, and 

 moving, as it does, from west to east, local rains cannot ordi- 

 narily take place within the same space for six or seven days, 

 on account of the scarcity of vapor in the air. 



Now, on these general laws, Professor Espy has founded a 

 curious and ingenious theory for the production of artificial 

 rains, which shall be general, extending all over the country in 

 times of drought. These, of course, would render the practi- 

 cable precautions which I shall have the honor to suggest of 

 comparatively little importance. I will give his theory in his 

 own words : — 



" Xow, if masses of timber, to the amount of forty acres for 

 every twenty miles, were prepared and fired simultaneously 

 every seven days in the summer, on the west of the United 

 States, in a line of six or seven hundred miles long from north 

 to south, then it appears highly probable from the theory, 

 though not certain until the experiments are made, that a rain 

 of great length, north and south, would commence on or near 

 the line of fires ; that this rain would travel towards the cast, 

 side foremost ; that it would not break up until it reaches far 

 into the Atlantic Ocean; that it would rain over the whole 

 country east of the place of beginning; that it would rain only 

 a few hours at any one place ; that it would not rain again un- 

 til the lower air becomes charged with vapor, and the upper 

 air has radiated off the heat which it received during the rain, 

 from the evolution of the latent caloric of the vapor, condensed 

 in the formation of cloud, which effects could hardly be pro- 

 duced in less than a week ; that it would rain enough, and not 

 too much, in any one place ; that it would not be attended with 

 violent wind, either on land or on the Atlantic Ocean; that 



