Contents xxvii 



PAGE 



the whole earth is at the rate of at least 109 x io 12 horse power. 

 Taking the radius of the earth's orbit to be 212 times the 

 radius of the sun, the radiation of one square metre of the 

 sun's surface is spread over 45,000 square metres of the earth's 

 surface ; therefore, the sun must radiate energy at the rate 

 of at least 37,000 horse power per square metre of its surface 372 



Comparison of the heat of the sun's rays with that pro- 

 duced by the combustion of liquid iron in oxygen . . . 373 



Observations made during the solar eclipse. 



Diagram of exposed surface of the sun at successive 

 epochs after totality : Fig. 9 376 



Table IV gives the rates of distillation observed at these 

 epochs 377 



Conclusion, containing remarks on the solar constant. 



The experimental value found with the steam calorimeter 

 at Sohag on i8th May 1882 was 0-89 gram-degree C. per 

 square centimetre per minute, without making any correction 

 whatsoever. It is certain that, using experimental means not 

 inferior in efficiency to the steam calorimeter, heat can be 

 obtained on the banks of the Nile from the sun at this rate. 



The only physical constant used is the latent heat of 

 steam, and this has been determined with the greatest 

 exactness. Therefore, the principle on which the instrument 

 is founded cannot be improved. The construction of the 

 instrument was by the late Mr John Milne of the Milton 

 House Works in Edinburgh, and no name in Great Britain 

 could be a better guarantee of exact workmanship : but, 

 independently of the reputation of the constructor, the instru- 

 ment did in fact do the work with perfect efficiency. In order 

 to save time in the construction, every part of the instrument 

 which could be so made was made out of brass tube. 

 Instrument-making is now so advanced that, with good will, 

 there should be no difficulty in getting a steam calorimeter 

 which, in competent hands, would give as good results as 

 I got. 



No. 11. SOLAR RADIATION. (From Nature, September 5, 1901, 



Vol. LXIV, p. 456 ; with postscript in 1911.) .... 383 



Only an infinitesimal proportion of the total radiation 

 reaches the earth. The remainder, in so far as we know, is 

 wasted by dissipation into space 383 



The amount of radiant heat which we can count on as 

 being supplied to the whole earth in unit of time is the 



