v.] 



SPRING FROSTS 



151 



between 3 and 4 p.m. on the second day was due to a 

 thunderstorm, during which heavy rain at a temperature 

 of 78 F. was falling. 



The figures in the table below are derived from 

 observations made by Bailey-Denton in 1857 (J. R. 

 Ag. Soc, 1859, 273), on a stiff clay soil situated on the 

 Gault at Hinxworth, the drains being 4 feet deep and 

 25 feet apart in the drained part. It is noteworthy 

 that the temperature of the air 9 inches above the 

 surface is higher for the drained than for the undrained 

 land, thus supplying further evidence of the cooling 

 effect of evaporation. 



Mean Temperature F. at 9 a.m. 



Effect of Situation and Exposure. 



Other conditions being equal, in the northern hemi- 

 sphere the soil temperatures will always be higher on 

 land sloping toward the southern quadrant than with 

 any other aspect. King found a difference of about 

 3 F. down to the third foot between a stiff red-clay soil 

 with a southern slope of 18 and the same soil on the 

 flat ; Wollny obtained a mean difference of i-5 between 

 the north and south sides of a hill of sandy soil inclined 

 at 15. The chief cause of these differences is the fact 

 that in this country the sun is never vertical, hence a 

 beam of sunlight represented by xy, Fig. 13, is spread 

 over an area represented by AB when the ground is 



