On Night-Frosts. 67 



From Ulis it will be secn that tluriiig a clear night the tcnipcrature of the tippciniost 

 laycr of cartli docs fall verv coiisiderably. It is evident that Ihis fall of tcnipciature nuist 

 be grcater cluring the tinic whcn the heating of the surfaec of the carth is niore intense. We 

 have hcre chosen the month of September, because, at that scasoii, the heating at 6,57 to 7,4 

 a. m. cannot be very perceptible. 



Knowing the specific heat of the laycr of carth aiul the average fall of tcmiicrattirc, a 

 calculatiou cau be made of tbe lost quantity of heat: 



Evoning. Moruiiig. 



70,4 4»,2 



If tbe lines given abovc dénote llie laycrs of carth at the dcptlis given in the mcasurcs, 

 we shall got the following: 



Thus, the tenipcrature bad fallen in 10 bours, at 0,4 m 0",?, at 0,2 ni 3",», and at 

 0,0 m 3»,2. 



Putting aside the more complicated phenomeua in the layers of earth from 0,2 m to 

 0,4 m wc shall find the surplus of température in tbe layer of earth [0,2 m — 0,n mj to bc, 

 in the cvening S'jS and, in the morning 4'',9 or in average 5",2 and the température, at the 

 surface of the earth, to be 5',8. 



Tbe température above the surface of the caith bad siraultaneously fallen frora 



3»,4 to 2",2 

 beiug thus in average 2",8. 



Tbe quantity of beat which, at a différence of température of P passes throngh 1 



cubic mater of quartz-sand in 1 hour, is, according to Peclet (Traite' de chaleur I, p. 554), 



0,27, consequcutly tbe wbole quantity of beat JJ/, providcd that Ihc conducting-powcr bc equal 



to that of tbe quartz-sand, 



,, 5,2 0,27 r, ^ , , 



M = _ = 7,02 Cal. per hour 

 0,2 



and thus, in 10 hours, 



70,2 Cal. 



