MARCH. 



109 



to the depth of three feet, it would supply sufficient moisture 

 for the whole months of June, July and August. 



"Rain is not the only source of supply of moisture to 

 plants; they absorb moisture from the air; and dews, which, 

 though they cannot be measured in the rain gauge, assist 

 very much in time of drought. But dews and atmospheric 

 moisture are not like rain — susceptible of being treasured up 

 in subterranean reservoirs, by deep tillage — and therefore 

 have not been taken into consideration in these investiga- 

 tions. It may, though not necessary to the inquiries now 

 before us, be well to observe that Bishop Watson made some 

 experiments that went to show that 'daring the time of 

 bright hot sun, when there had been no rain for a month, the 

 evaporation from grass was at the rate of .035 inches in twelve 

 hours. Another experiment, one day after a thunder storm, 

 gave .087 in twelve hours. The mean is .061 inch.' The 

 evaporation of .035 in twelve hours in a drought of a month's 

 continuance, gives, allowing the evaporation to go on only 

 during twelve hours in each day for a month, 1.05 inches. 

 This must have been principally supplied to the plants dur- 

 ing nights, in the form of vapor and dew, as it is not proba- 

 ble, after a month without rain, that earth tilled to the ordi- 

 nary depth would afford much more moisture than it received 

 from these sources. 



"It is calculated in England that the ordinary summer run 

 of streams does not exceed ten cubic feet per minute per 

 square mile, and that the average for the whole year due to 

 springs and ordinary rains is twenty feet per minute per square 

 mile exclusive of floods, and assuming no very wet or high 

 mountain districts, (Breadmore, p. 34,) which is equal to 

 about four inches over the whole surface. If we add to this 

 the six inches that are supposed to run off' in freshets, we 

 have ten inches discharged in the course of the year by the 

 streams. The whole filtration was 11.29 inches, 10.39 in 

 the winter, and .90 in the summer. The remainder, 1.29 

 inches, is supposed to be consumed by wells and excessive 

 evaporation from marshes and pools, from which the dis- 

 charge is obstructed ; by animals, and in various other ways. 



