SECRETARY'S REPORT. 73 



Amount evaporated, in inches and hundredths, from the soil. 



First Day. Second Day. Third Day. Fourth Day. Fifth Day. Sixth Day. Seventh Day. 

 .1614 .0984 .0708 .0511 .0511 .0472 .0511 



The amount of evaporation from water, at the same time, was, 



First Day. Second Day. Third Day. Fourth Day. Fifth Day. Sixth Day. Seventh Day. 

 .59 .53 .45 .47 .46 .43 .37 



This shows how rapidly evaporation takes place from the 

 surface after a rain, more than a fourth of the whole amount of 

 moisture in the soil being raised the first day. As soon as the 

 earth becomes dry, the evaporation is scarcely appreciable. 

 At the end of the second day, at the temperature given above, 

 the earth will generally have become dry to the depth of from 

 .07 to .11 in. ; and after eight days, plants which do not penetrate 

 the soil more than four inches will begin to suffer from drought, 

 and require watering. In the higher temperatures which we 

 frequently have, evaporation is more rapid. But if a light rain 

 falls on a hard surface, it cannot penetrate to any great depth ; 

 while, on a light and porous soil, it is absorbed at once, and 

 will not evaporate so rapidly, since it is in some measure pro- 

 tected by the earth. So with dews. We have seen that on a 

 hard surface there is no dew, or but little ; and even if there 

 were much, it would very soon evaporate ; while on a mellow, 

 porous surface, much more is deposited, for the reason inti- 

 mated above, and it must remain longer in the soil. It is a 

 curious fact, also, that the most thoroughly drained land stands 

 the drought best. This is explained by the consideration that 

 the roots of plants can shoot down to a greater depth in such 

 land without finding substances injurious to them. 



Now, our summer rains ordinarily come at considerable inter- 

 vals, and are immediately followed by scorching suns or drying 

 winds. Still more frequently they come in showers, followed 

 by an atmosphere as clear and unclouded as any in the world. 

 If the rain has penetrated through a light, porous soil, it may 

 evaporate sooner than the same amount of moisture distributed 

 through the same depth of a hard and impermeable surface ; 

 but we have seen that the same amount of rain does not pene- 

 trate the hard surface, or, if at all, only to a partial extent, 



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