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, 5*1 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 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 0 84 horse-power per sq. metre, then, as the area of 
a great circle on the earth’s surface is 1 29*9 x 10 12 sq. metres, the 
useful energy received by the whole earth is at the rate of 
i 
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. 
