68 Mr Buchanan, On a solar Calorimeter used in Egypt 



of water at 100° C. 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, 51 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 9035 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 16*6 grs. of steam at 100° C. 

 Therefore by the use of ordinary mechanical appliances it is possible 

 under favourable geographical and meteorological conditions to 

 collect on a square metre of surface exposed perpendicularly to the 

 sun's rays the energy of generation of 16 '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 work- 

 ing 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 084 horse-power per sq. metre, then, as the area of 

 a great circle on the earth's surface is 1299 x 10 12 sq. metres, the 

 useful energy received by the whole earth is at the rate of 



109 x 10 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. 



Observations during the Eclipse. 



The calorimeter was directed to the sun as soon after totality 

 as possible. At 8 hr. 34 min. the sun was totally eclipsed; at 

 8.51 the calorimeter was directed to the sun but no boiling took 

 place. At 8.58 the water began to " sing " ; at 9.1 it boiled ; at 

 9.3 it was boiling briskly, but it was not till 9.17 that the first 

 drop of distillate fell into the receiver. By 9.19*5 1 c.c. had 

 passed, and between 9.21 and 9.29*5 5 c.c. passed. 



