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. 
[. Received 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. 
