66 Selim Lemström. 



earth and generally comes in from all directions, and, in tlie second place, the température 

 of the radiating surface falls, by wliich the surplus of température t is rapidly diminished. 

 Hence, on calculatiug the quantitties of heat, we shall find that o and t are both variable. 

 However, to give an idea of this quantity of heat, we will proceed from the following hypoth- 

 èses, deduced from the observations below. 



As the bare ground, not overgrown with grass, is during clear nigbts coated with hoar- 

 frost very shortly after sun-set, we must impute to all species of earth a relatively small 

 conducting power for heat, and we shall understand, at the same time, that the température, 

 having fallen to the dew-point, the radiation of heat will issue from a surface consisting al- 

 most entirely of water in liquid or solid form. 



Assuming therefore. 



Z=5,31 



corresponding to water and according to the observations 



o = O and ; = 00,2 

 we shall get 



l,ofi Cal. per hour and per m^. 



This estimation may yet be too high, because for sand 



K — 3,32 



and may, in this case, be between the worth of sand and that of water. 



As the température of the upper layer of earth falls very much during a clear night, 

 this circumstance bas been given as a reason for the assertion that the whole quantity of 

 heat, represented by 



P C T 



for the layer of earth to aTsquare-nieter, woiild radiate from the surface of the earth. 



[P denoting the wcight of this layer, C its specific heat and T the average number of 

 degrees the température bas fallen]. As an example We give from the observations at Paw- 

 low sk : 



18 ??-^ 90 



Radiation Temp. of the 



Date. Time. tliermometer. Sky. Wind. Surf of earth. Time. Temp. of eartli at a dept.h of. 



Sept. b. m. h. m. 0,o m 0,2 m 0,4 m 0,ö m 



20 8,57 p. m. 100,5 clear. O 6»,9 9.4 p. m. 9»,2 



21 6,57 a. m. 11,6 es W 5,8 7,4 a. m. 6,9 



21 8,57.p. m. 7,9 se S 5, 9,4 p. m. 9,o 



22 6,57 a. m. 8,0 S 3, 4 7,4 a. m. 5,o 

 22 8,57 p. m. 5,2 - WSW 2,2 9,4 p. m. 7,4 

 21 6,57 6,7 — — 2, 4 7,4 4,2 



