INTERCHANGE OF HEAT VON BEZOLD 403 



does not fall below the freezing point, then the total stored up 

 energy is present only in the form of heat that can be measured 

 thermometrically. 



If the ground contains water and if the temperature passes the 

 freezing point in either one direction or the other, then the relations 

 become more complicated, but still problems relating even to 

 these cases are much simpler than most of the others that occur 

 in reference to the subject here treated. 



Moreover, as will be seen later, this matter [of freezing] does not 

 at all come into consideration, at least in lower and middle latitudes, 

 in the determination of the quantity of heat taken in and given out 

 during an annual period and independent of the diurnal exchange. 



To attain our present object the important matter is the solution 

 of the two following questions: 



(1) How great is the difference between the quantities of heat 

 taken in and given out by a unit area of surface during a given 

 interval of time, that is to say, how great is the increase or decrease 

 of energy experienced by the earth beneath that unit of surface 

 during this interval of time? 



(2) How great is the difference between the maximum and 

 minimum values of the energy present in this portion of earth dur- 

 ing a given interval of time? 



The reply to the first of these questions will give the energy stored 

 in the earth during a given interval of time or the quantity present 

 therein at any moment. 



The reply to the"" second question gives us a measure of the 

 efficiency of the ground as a regulator of heat, provided we choose 

 the interval to be studied so that it includes a complete period of 

 thermal-exchange, such as a whole day or a whole year. 



The reply to these two questions is extremely simple, as will be 

 shown immediately, since it only assumes a knowledge of the tem- 

 peratures at different depths and of the capacity for heat of the 

 unit volume of earth, the so-called volume capacity 12 whereas the 

 conductivity of the earth as well as the radiation or emission at 

 the surface do not come into play. 



Moreover, in the solution of the second question, it suffices if we 

 know the earth temperature for that day or season at which the 

 temperature gradient in the highest layer of earth is zero. 



Easy as it would be to answer these questions, and important as 



As distinguished from the specific capacity which relates to the unit mass. 



C. A. 



