No. II. [From Nature, September 5, 1901, Vol. LXIV, 

 p. 456; with postscript in 1911.] 



SOLAR RADIATION 



SOLAR radiation is a subject which has more than scientific 

 interest. It is the source of all the energy which maintains 

 the economy of our globe. It lights and heats the other 

 members of the planetary system. But, after accomplishing 

 this, only an infinitesimal proportion of the total radiation 

 has been used. The remainder, in so far as we know, is 

 wasted by uninterrupted dissipation into space. 



The subject can be regarded and studied from either the 

 solar or the terrestrial point of view. In terrestrial physics 

 everything may be said to depend on the energy which, in 

 one form or another, is supplied by the sun's rays. It is the 

 revenue of the world, and it is of fundamental importance for 

 us to know at what rate it falls to be received. 



Roughly speaking, the surface of the earth is occupied to 

 the extent of one-fourth by land and three-fourths by sea. 

 Therefore at least three-fourths of the surface which the 

 earth presents to the sun is at the sea-level. Consequently 

 the rate at which the sun's radiant heat arrives at the 

 sea-level is the fact which it is of the greatest economical 

 importance to ascertain. 



In considering this problem we have to answer two 

 questions : What is the best experimental method of deter- 

 mining the heating power of the sun's rays at any place ? 

 and What is the best locality for making the experiment ? 

 Let us take the last first. The energy which a radiation 

 communicates to a surface is greatest when it strikes it 

 perpendicularly. At every moment the sun is vertical over 



