74 Mr Buchanan , On a solar Calorimeter used in Egypt, etc. 
collect from a square metre of surface exposed perpendicularly to 
the sun’s rays 8888 gr.° C. of heat which are equivalent to 
3777 kgm. of work per minute or 0’84 horse-power. If we allow 
16 per cent, for losses from all causes the result is one horse-power 
received from the Sun by every square metre of the area included 
in a great circle of the Earth which is roughly 130 x 10 12 sq. 
metres and this figure in horse-power represents the working value 
of the Sun in its relations to the Earth. Accepting the value of 
one horse-power per sq. metre at the distance of the Earth we 
find by simple arithmetic that the working value of 1 sq. metre 
of the sun’s surface must be 45000 horse-power. It follows that 
the area of the sun’s surface which we may regard as hypo- 
thecated to the earth’s heat service is no more than 2900 square 
kilometres which would be contained in a circle of 60 kilometres 
diameter and would subtend an angle of at most one tenth of a 
second. As over five hundred millions of such areas are included 
in a great circle of the Sun, it is clear that the maintenance of the 
Earth’s heat is well assured. 
