Action of the Atmosphere upon newly-deepened Soil. 409 



Reaching therefore the crust of the earth, the force of the sun's 

 rays will vary according to the exposure of the surface ; if its in- 

 clination be such that the rays strike perpendicularly upon it, 

 then the soil will be twice as much heated as it would be were 

 the same amount of sunshine, by coming obliquely upon it, spread 

 over double the surface. Much also will depend upon the colour 

 of the soil, for it has been clearly proved that, for surfaces of the 

 same texture, the heating effect of the sun's rays is in proportion 

 to the darkness of the colour ; this may be shown in a simple 

 manner by placing differently coloured pieces of cloth on the 

 snow exposed to the sun, when the darkest bits will be seen 

 to sink fastest. 



The ground, being generally of a dark colour, may, if dry, be 

 expected to attain, when the sun beats upon it, a temperature 

 similar to what a thermometer with a blackened bulb would 

 under similar exposure. Let us see, therefore, what that 

 may be. 



Saussure, by properly defending the thermometer from wind 

 and common radiation, raised the ten^perature, in the sun, to 

 190° F. Pouillet also states that he found the thermometer, 

 properly protected and exposed perpendicularly to the solar rays, 

 assume considerable elevations, often rising 90° F. above the 

 ambient temperature towards noon ; it sometimes even rose to 

 194*^ F., the temperature of the air being 80^.6 F., showing an 

 elevation in this case of 113°.4 F. ; and as the result of his 

 observations he found that the thermometer exposed to the sun 

 may, according to the dispositions given it, take any required 

 excess above the temperature of the air from 5"* F. up to 115° F. 

 (Scientific Memoirs, vol. iv., pt. 13, p. 90.) Even at Edin- 

 burgh in July Mr. Foggo obtained by means of a large ther- 

 mometer having the ball covered by dark wool and fully exposed 

 to the direct rays of the sun, unsheltered from the wind, a tem- 

 perature of 150°. 



We may therefore expect that in calm, clear, summer weather 

 the surface of the ground will attain a very high degree of heat, 

 and tliat this will espcciallv be the case under a vertical sun. 

 In \ul)ia, "under a hot and copper sky," the Arabs say the 

 soil is like fire. Sir John Herschel, on the 5tli of December, 

 1837, between one and two p.m., observed the heat under the soil 

 of his bul!)-garden at the Cape of Good Hope to be 159^ F., at 

 3 r.M. it was 150°, the temperature of the air in the shade 

 in the garden at the sauK? time being 98° and 92°. ((Quoted bv 

 Lindley in liis Principles of Ilortirnltitrc, p. 125.) 



01)servations are much wanted to show what temperature the 

 surface soil attains in this country. Mr. Whitley, of Truro, so 

 late in the season as the l()th of September, tlie air in the shade 



