RELATIONS OP THE SOIL TO HEAT. 197 



with the deepening of the color, until in case of humus it 

 lacks but a few degrees of reaching the warmth of a sur- 

 face of lamp-black. 



According to the observations of Dickinson, made at 

 Abbot's Hill, Ilertfonlshire, England, and continued 

 through eight years, 90 per cent of the Avater falling be- 

 tween April 1st and October 1st evaporates from the sur- 

 face of tlie soil, only 10 per cent finding its way into 

 drains laid three and four feet deep. The total quantity 

 of water ^hat fell during this time amounted to about 

 2,900,000 lbs. per acre; of this more than 2,000,000 evap- 

 orated from the surface. It has been calculated that to 

 evaporate artificially this enormous mass of water, more 

 than seventy-five tons of coal must be consiimed. 



Thorough draining, by loosening the soil and causing a 

 rapid removal from below of the surplus water, has a most 

 decided influence, especially in spring time, in warming 

 the soil and bringing it into a suitable condition for the 

 support of vegetation. 



It is plain, then, that even if we knew with accuracy 

 ■what are the physical characters of a surface soil, and if 

 we were able to estimate correctly the influence of these 

 characters on its fertility, still we must investigate those 

 circumstances which afiect its wetness or dryness, whether 

 they be an impervious subsoil, or springs coming to the 

 surface, or the amount and frequency of rain-falls, taken 

 in connection with other meteorological causes. We can- 

 not decide that a clay is too wet or a sand too dry, until 

 we know its situation and the climate it is subjected to. 



The gre'at deserts of the globe do not owe their barren- 

 ness to necessary poverty of soil, but to meteorological 

 influences — to the continued prevalence of parching winds, 

 and the absence of mountains, to condense the atmospheric 

 water and establish a system of rivers and streams. This 

 is not the place to enter into a discussion of the causes 

 that may determine or modify climate ; but to illustrate 



