INTERCHANGE OF HEAT VON BEZOLD 377 



direction by Woeikof 3 and similarly in a recent highly interesting 

 memoir by W. Trabert. 4 



Attention has hitherto to a large degree been given to only one 

 side of this problem, namely, the theory of insolation or the radia- 

 tion from the >un to the earth and that of the radiation from the 

 earth into space — a chapter on which, as is well-known, there is an 

 extensive literature. 



But although it must be allowed that exact determinations of 

 these two elements are among the most important points of the 

 whole question, still we ought not to forget that it is precisely here 

 that we meet the greatest difficulties unless we restrict ourselves 

 to purely theoretical considerations as J. H. Lambert, L. W. Meech, 

 and Christian Wiener have done. 



Recently O. Chwolson 5 has clearly shown how important are 

 the difficulties and how large the corresponding uncertainty that 

 still attends this field of work in spite of all the thought that has 

 been bestowed upon it. 



That the degree of accuracy that has hitherto been attained in the 

 determination of the intensity of the solar radiation is still quite 

 moderate is seen moreover from the simple fact that we cannot 

 yet recognize the change from perigee to apogee, although it must 

 amount to one-fifteenth or 7 per cent of the total amount. 



In consideration of the difficulties offered by the solution of this 

 apparently simple question, and in view of the uncertainty that still 

 exists with reference to the most important constants, it might 

 indeed seem premature to attempt to extend the investigation 

 over the far more complicated process through which the heat 

 furnished by the sun has to pass, from its entrance into the atmos- 

 phere until its exit therefrom into celestial space. 



But still this labor must be undertaken. We must attempt to 

 determine, at least approximately, what fraction of the heat which 

 comes into play at any part of the earth's surface or of the atmos- 

 phere, in a given time, is furnished by direct insolation and lost by 

 direct radiation; how much is brought hither and removed hence by 

 simple or complex convection; how r much is used in evaporating 

 water or melting ice; how much is stored in the ground only to be 



3 Woeikof: The climates of the Globe. Jena, 1887. 



4 Trabert: The diurnal variation of temperature and sunshine on the sum- 

 mit of Sonnblick. Memoirs of the Vienna Acad. Math. Classe, Vol. LIX, 

 1892. 



5 Chwolson: On the present condition of actinometry. Wild Repertor- 

 ium, 1892, XV, No. 1. 



