V20 ANNUAL REPORT OF NEW-YORK 



to speculate upon this topic. The rapidity of cooling appears to 

 stand in direct connection with the lightness and porosity of the 

 soil; such qualities favor radiation, and the loss of heat by the 

 circulation of the inclosed air. Such soils also, in nature, enclose 

 a considerable amount of water, and in them capillary action is 

 strongest in raising supplies from the sub-soil * On account of 

 their porosity, this water is constantly evaporating, and therefore 

 by extracting from them the heat necessary to vaporization, their 

 temperature is speedily reduced. For the same reason moist soils 

 rich in humus, can warm but slowly in the sun's rays. Sandy 

 soils retaining little water, evaporation is less active in re- 

 ducing their temperature. The surfaces of the grains of sand 

 are glassy and smooth, they therefore radiate poorly, though this 

 depends of course on the degree of fineness and smoothness. Clays 

 stand in the middle of the scale. 



3. The degree of moisture present is of great influence on the 

 temperature of the soil. All soils when thoroughly wet seem to 

 be nearly alike in their power of absorbing and retaining w^armth. 

 The vast quantity of heat needful to gratify the demand of the 

 vapor that is constantly forming, explains this. From this cause 

 the difference in temperature between dry and wet soil may often 

 amount to 10^ to 18^. According to the observation of Dickin- 

 son made at Abbot's Hill, Herts, and continued through eight 

 years 90 per cent, of the water falling between April 1st and 

 October 1st. evaporates from the surface of the soil; only 10 per 

 cent, finding its way into drains laid three and four feet deep. 

 The total quantity of w^ater that fell during this time, amounted to 

 about 2,900,000 lbs. per acre; of this more than 2,600,000 eva- 

 porated from the surface. It has been calculated that to evapo- 

 rate artificially, this enormous mass of water, more than seventy- 

 five tons of coal must be consumed. 



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 fertil- 

 ity, still we must investigate those circumstances which affect its 



