214 IMPOETANCE OF SUMMER KAIIS'S. 



This conclusion, however appHcable to the climate and to the 

 soil of France, is too broadly stated to be received as a general 

 truth ; and in countries hke the United States, where rain is com- 

 paratively rare during the winter and abundant during the sum- 

 mer half of the year, common observation shows that the quantity 

 of water furnished by deep wells and by natural springs depends 

 almost as much upon the rains of summer as upon those of the 

 rest of the year, and consequently that a large portion of the rain 

 of that season must find its way into strata too deep for the water 

 to be wasted by evaporation.* 



vicinity of Copenhagen, where the mean annual precipitation is 33| inches, 

 and where the evaporation must be less than in the warmer and drier atmos- 

 phere of France, form one of the most careful series of observations on this 

 subject which I have met with. Johnstrup found that at the depth of a metre 

 and a half (59 inches) the effects of rain and evaporation were almost imper- 

 ceptible, and became completely so at a depth of from two to three metres {Qi 

 to 10 feet). During the summer half of the year the evaporation rather ex- 

 ceeded the rainfall ; during the winter half the entire precipitation was absorbed 

 by the soil and transmitted to lower strata by infiltration. The stratum be- 

 tween one metre and a half (59 inches) and the three metres (10 feet) from the 

 surface, was then permanently in the condition of a saturated sponge, neither 

 gaining nor losing humidity during the summer half of the year, but receiving 

 from superior, and giving off to lower, strata an equal amount of moisture 

 during the winter half. — Johnstrup, Om Fugtighedens Bevmgelse i den no- 

 turlige Jordbund. Kjobenhavn, 1866. 



Dalton's experiments in the years 1796, 1797 and 1798 appeared to show that 

 the mean absorption of the downfall by the earth in those years was twenty - 

 nine per cent. 



Dickinson, employing the same apparatus for eight years, found the absorp- 

 tion to vary widely in different years, the mean being forty-seven per cent. 



Charnock's experiments in two years show an absorption of from seventeen 

 to twenty-seven per cent. 



Risler, in experiments referred to on p. 187, ante, found that no water escaped 

 from a parcel of ground not far from Geneva, thoroughly underdrained to the 

 depth of 1"" 20, in the months of July, August, September, November and De- 

 cember, 1867, and but a very trifling quantity in June and October of the same 

 year ; in 1868, very little in May and September, and none in June, July and 

 August. In 1867, the total precipitation was 1066.84 millimetres ; the evapo- 

 ration, as measured by the difference between rainfall and drainage, 733.44 

 millimetres ; in 1868, these quantities were 1032,86 and 755.74 millimetres re- 

 spectively. 



* According to observations at one hundred military stations in the United 

 States, the precipitation ranges from three and a quarter inches at Fort Yuma 

 in California to about seventy-two inches at Fort Pike, Louisiana, the mean 



