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 suns 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. 



