INTERCHANGE OF HEAT VON BEZOLD 389 



tion and radiation" or "lines of radiation equilibrium" or briefly 

 "neutral lines." 



There are two such neutral lines where the radiation outward and 

 inward balance each other, one of which belongs to the northern and 

 the other to the southern hemisphere. But it is not incredible that, 

 besides these, other similar closed lines may exist that must be 

 regarded as boundaries for smaller regions, like islands. 



If we give the positive sign to the heat radiated from the sun to 

 the earth and the negative sign to that radiated by earth to space, 

 then the algebraic sum of the quantities of heat exchanged through 

 the boundary surface of the atmosphere is positive within the equa- 

 torial zone but negative in the two polar zones. We can, therefore, 

 as to annual averages, think of the whole exchange of heat within 

 the atmosphere and at the earth's surface as schematically repre- 

 sented by a current of heat that enters into the equatorial zone 

 through the boundary surfaces of the atmosphere and after splitting 

 into two branches departs in the polar zones. 



The location of the neutral lines for the balance of radiation and 

 the determination of the intensities of these ideal streams, i. e., 

 the quantities of heat that are interchanged in this way, forms an 

 important problem in that chapter of the physics of the atmosphere 

 that we have now under consideration. 



In fact we have not to do with such a simple single flow of heat 

 but with double currents, since warm masses are simultaneously 

 carried poleward and cold masses equatorward, whose sum 

 total represents the simple current of the above scheme. Therefore 

 the considerations to be here set forth have a certain analogy with 

 those by which one passes from the assumption of a double cur- 

 rent over to that of a single current as in the case of the double and 

 single current theories in electricity. 



The theorems just stated may be algebraically expressed in the 

 following forms: 



(8) 



Q > Q in the equatorial zone 



Q < Q in the polar zones 



Q = Q . . . . along the two neutral lines 



which latter may be represented by the equations 



(+ fit) - and ¥(- (3X) = 



where /? expresses the absolute value of the latitude, and we reckon 

 northern latitudes as positive and southern latitudes as negative. 



