Action of the Atmosphere upon neicly- deepened Soil. 415 



at Kew Observatory. As the sky was not all clear, a greater 

 difference would doubtless be observable under more favourable 

 circumstances. 



Mi\ Six, of Canterbury, also found the thermometer, which had 

 been suspended in the air on a clear still night in winter, fall 13i° 

 when placed flat on a meadow ; and Scoresby and Capt. Parry 

 observed analogous depressions in the Polar regions when the 

 temperature of the air was more than 20° below zero. Wells 

 noticed a difference of two thermometers, one in the air the other 

 placed in swan's-down (a good radiating substance), amounting to 

 15° F. {Brit. Assoc. Report, 1840, p. 65.) 



M. Pouillet, from observations made in clear nights in April, 

 May, and June, with thermometers placed in swan's-down, found 

 the difference arising from radiation to be from about 10° F. to 

 16° F. below the temperature of the air at 4 feet from the ground ; 

 the greatest difference noticed being 16°.2 F. on the 5th May, 

 between eight and nine in the evening. The temperature lowered 

 gradually during the night till it attained its minimum towards 

 sunrise. The thermometer so exposed to radiation fell frequently 

 below the freezing point of water during these observations, 

 whereas the one hung in the air never was below 32° F. 

 {Scientijic Memoirs, vol. iv. part 13, 1844.) With regard to the 

 radiation of heat by the earth to air. Prof. Marcet has arrived at 

 the following conclusions : — 



1. It is a constant phenomenon about the time of sunset, except 

 in tiie case of violent winds. 



2. It attains its maximum immediately after sunset. 



3. It is most conspicuous when the ground is covered with 

 snow. 



In the Andes, Darwin oljscrved that where the rock was covered 

 during the greater part of the year with snow, it was shivered in 

 a very extraordinary manner into small angular fragments. And 

 this lowering of the temperature of the ground does not occur 

 merely on still clear nights, but also, as I have repeatedly ob- 

 served, takes place by evaporation in windy weather. Thus in 

 December, 1855, during very windy weather from the south-east, 

 when the register thermometer did not fall below 34^.3 1\, all 

 the j)uddles and wet mud on the roads were frozen hard, the sky 

 being cloudy. The amount of cokl due to evaporation is often 

 very considerable. Thus at Southampton, on PJth April, 1854, 

 at 3 P.M., Mr. Drew oljserved a difference of 16° between the 

 readings of the wet and dry bulb tiu>rmometers, the one being 69^ 

 and the otiier 53°. A nearly cc{ual difference was noticed by 

 Kamtz on the summit of the FauUiorn, the dry bull) l)eing at 

 47° 1"". and tlie wet at 32-'. At Geneva, in August, 1832, Prof. 



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