68 Selim Lemstköm. 



The volume of the layer of earth is 



200 cubic decimeter, 



Its specific heat is not known with certainty, but as a layer of earth contaius water to 

 about 30 o/oi this quautity, if supposed = 0,5, will not be too lowly estimated. 

 From this foUows, for every degree thc température falls, a loss of 



100 Cal. 



and for 5'',2 



520 Cal. 



Thongh, indeed, this worth is to be considered as a maximi worth, yet the contradic- 

 tion it implies with regard to 



a) the experiments mentioned above 



b) the above-mentioned quantities of beat found by conducting, is so great that there 

 mtist be some partimdar cause of it. 



This cause is not difficult to find, consisting in the evaporation from the uppermost 

 layer of earth. 



We assume the ground in question to be, as is always the case, more or less porous 

 and consequently crossed by tubes or canals. As soon as the température at the surface 

 of the earth begins to fall and the uppermost layer of earth to cool down, the warmer and 

 consequently lightcr air and the aqiieons vapour in it miist rise, thus furthering the evapo- 

 ration below and, in this way, a considérable consumption of beat. The rising vapour is 

 Condensed more and more the higher it rises, and in the surface of the earth its tension 

 must be roduced to a worth corresponding to the température prevailing in the lowest layers 

 of air. The uppermost part of the surface of the earth and the lowest layer of air, bave, in 

 fact, vcry ncarly, thc samc température. (Peclct 1. c. page 577 — 579). 



In the above-mentioned case the tension of the aq. vapour, which at a depth of 0,2 m was 



11,0 mm corresp. to 12'',9 

 bas, at the surface of the earth, a tension of 



7,0 mm corresp. to 7",4 

 and bfeore it passes into thc air, only its tension having thus becu reduccd with 5,6 mm. 



5,4 mm corresp. to 2'',3 



It is difficult to détermine the quantity of water which évaporâtes from such a layer of 

 earth, for in this case we hâve no exact experiments to rcfcr to. Expérience tells us that 

 this quantity is by no means insignificant, and that it will be the greater, the more porous 

 the earth is, which is seen from the experiments made rcspecting the quantity of dew, about 

 which more will be said below. 



Assuming that the layer of earth loses in 10 hours one kg of aq. vapour correspon- 

 ding to 0,5 "/o of its volume, it will, at every moment, contain a quantity of vapour of the 



average tension 



9,85 mm (awer. temp. 10'',i5.) 



