37 2 On a solar Calorimeter used in Egypt 



water as unity, and the mechanical equivalent of heat as 

 0-425 kilogramme-metres per gramme-degree. Therefore the 

 heat required to transform 1*5 grs. of water at iooC. into 

 steam of the same temperature is 803 gr. C., and this is the 

 greatest amount of heat which the calorimeter has recorded 

 in one minute. On careful measurement of the calorimeter, 

 especially the reflector, I find that its actual collecting 

 diameter is 34/3 centimetres, less that of the condenser tube, 

 5 'I centimetres. So that its collecting area is 



924 20*5 = 903^5 square centimetres (cm. 2 ). 



Therefore the rays of the sun falling perpendicularly on a 

 surface of 903-5 cm. 2 supplied it with heat at the rate of 

 803 gr. C. per minute. This is equivalent to 8888 gr. C. 

 per square metre ; and 8888 gr. C. suffice for the generation 

 of 1 6*6 grs. of steam at 100 C. Therefore by the use of 

 ordinary mechanical appliances it is possible under faiwurable 

 geographical and meteorological conditions to collect on a square 

 metre of surface exposed perpendicularly to the sun's rays the 

 energy of generation of :6'6 grs. of steam per minute. But 

 8888 gr. C. of heat are equivalent to 3777 kgm. of work ; and 

 this work is done in one minute, therefore the agent is working 

 at the rate of at least 0^84 horse-power. 



The agent is the energy of the sun's rays which fall upon 

 a surface of one square metre, exposed perpendicularly to 

 them at the distance of the earth. If the sun throws so much 

 radiant energy that it can be collected and utilised at the 

 earth's surface at the rate of 0-84 horse-power per sq. metre, 

 then, as the area of a great circle on the earth's surface is 

 129-9 x IC)12 sc l- metres, the useful energy received by the 

 whole earth is at the rate of 



109 x io 12 horse-power. 



Taking the radius of the earth's orbit to be 212 times the 

 radius of the sun, the radiation of one sq. metre of the sun's 

 surface is spread over 45,000 sq. metres of the earth's surface ; 

 therefore the sun must radiate energy at the rate of at least 

 37,000 horse-power per sq. metre of its surface. 



