Mr Buchanan, On a solar Calorimeter used in Egypt, etc. 37 



On a solar Calorimeter used in Egypt at the total solar Eclipse 

 in 1882 1 . By J. Y. Buchanan, F.R.S. 



[Eeceived 15 December 1900.] 



While engaged in discussing questions connected with the 

 physics of the ocean, I found the want of definite knowledge 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 perpendicularly 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 experiments 

 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 expedition 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 favourable circumstances. I determined 

 to use a calorimeter which should depend for its indications on 

 change of state and not on change of temperature. The ice 

 calorimeter naturally suggested itself; but apart from the fact 

 that in 1882 ice was not so universally procurable as it is now, 

 the indications of the ice calorimeter are apt to be seriously 

 modified by the condensation of moisture from the air. I there- 

 fore determined to make a steam calorimeter in which the sun's 

 rays should be collected by a conical reflector of definite area and 

 thrown on an axial tube which should represent the boiler. 



Locality. The astronomers had fixed on a spot on the banks of 

 the Nile close to the town of Sohag and in latitude 26° 37' N. for the 

 observation of the eclipse, and experience showed that it had been 



1 See Proceedings of the Royal Society of Edinburgh (1882), xi. 827. 



