204 



On the Physical Properties of Soil, 



the capacity of soils to become warmed by the sun^ than their 

 colour and dryness. If we impart to earths artificially the same 

 colour^ and expose them in a similar state of dryness to the heat 

 of the siin^ the differences of temperature will be inconsiderable ; 

 so that the differences in this respect shown by the several earths 

 in their natural state may be referred in a particular manner to 

 these two leading circumstances^ colour and dryness. 



The following table contains the results of a series of experi- 

 ments which I made on the different degrees in which earths ac- 

 quire warmth from the sun in fine weather. I placed these earths 

 in vessels of four square inches in surface and half an inch deep, 

 and exposed them to the rays of the sun, coloured differently on 

 the surface, and furnished with thermometers as already described; 

 the observations were made in the latter part of August, and be- 

 tween 1 1 and 3 o'clock in the day, while the temperature of the 

 air varied in the shade from 72^° to 77° F. As all the observa- 

 tions could not be made at once, the temperature which sand ac- 

 quired on the same occasion was in each case taken as the standard 

 of comparison, to which all the several observations have been 

 reduced. 



Kinds of Earth. 



Mean of Highest Temperature of the upper sur- 

 faces of the Earths. (77° F. in the shade.) 



With a surface of the 

 natural colour. 



With dry earth. 



Wet. 



Dry. 



With a white 

 surface. 



With a black 

 surface. 



Siliceous Sand, bright yellowish-grey 



o 



99.1 



112.6 



109.9 



123.6 



Calcareous Sand, whitish-grey . 



99.3 



112.1 



109.9 



124.0 



Gypsum, bright white-grey . . 



97.3 



110.5 



110.3 



124.3 





98.2 



111.4 



108.3 



121.6 



Loamy clay, yellowish .... 



99.1 



112.1 



107.8 



121.1 



Stiff' clay, or brk. earth, yellowish- grey 



99.3 



112.3 



107.4 



120.4 



Fine bluish-grey clay .... 



99.5 



113.0 



106.3 



120.0 





96.1 



109.4 



109.2 



122.9 





95.2 



108.7 



108.7 



121.3 



Humus, brownish -black .... 



103.6 



117.3 



108.5 



120.9 



Garden-mould, blackish-grey . 



99.5 



113.5 



108.3 



122.5 





97.7 



111.7 



107.6 



122.0 



Slaty marl, brownish-red 



101.8 



115.3 



108.3 



123.4 



Influence of the Inclination of the Ground 07i the amowit of 

 Warmth it acquires from the Sun. — The inclination of the ground 

 towards the sun has a very considerable influence on the degree 

 of warmth which the soil receives from its rays ; and the amount 

 of warmth so produced is, under similar circumstances, always 

 greater the more nearly the incidence of the ray approaches to a 



