No. 10. [From the Proceedings of the Cambridge Philosophical 

 Society, 1901, Vol. XI, Pt /, //. 37-74.] 



ON A SOLAR CALORIMETER USED IN 

 EGYPT AT THE TOTAL SOLAR 

 ECLIPSE IN 1882' 



WHILE engaged in discussing questions connected with 

 the physics of the ocean, I found the want of definite know- 

 ledge of the amount of solar heat which really reaches the 

 surface of the land or sea in a form which can be collected, 

 measured and utilised. There was no lack of actinometrical 

 observations, but I found it impossible from them to obtain 

 the data that I sought. The aim of most observers has been 

 to arrive by more or less direct means at what is known as 

 the solar constant, that is, the quantity of heat which is 

 received in unit time by unit surface when exposed per- 

 pendicularly to the sun's rays outside of the limits of the 

 earth's atmosphere. For my purpose the radiation arriving 

 at the outside of the earth's atmosphere was of no importance. 

 What I did want to know and to measure was the amount of 

 solar radiation which strikes the earth at the sea-level and is 

 there revealed as heat. It is the energy of this radiation 

 which maintains the terrestrial economy. I was not satisfied 

 with the values of it which could be deduced from the experi- 

 ments which had been made with the view of ascertaining the 

 value of the solar constant, and I determined to utilise the 

 opportunity of a visit to Egypt in company with the expedi- 

 tion for observing the total eclipse of the sun in 1882 to 

 make observations for myself on the amount of heat which 

 could actually be collected from the solar radiation in these 



1 See Proceedings oj * the Royal Society of Edinburgh (1882), vol. xi. p. 827. 

 B. 22 



