148 THE ANTARCTIC MANUAL. 
in order to make the example complete, and not as an experimental 
determination of the calorific power of the sun’s rays. 
At the same time it illustrates the principle of the actinometric 
method of determining the heat of the sun’s rays, which has been 
much used. 
Apart from the annual cycle of variation of the sun’s distance 
from the earth, there is no reason to expect any measurable 
variation in its heating power. Therefore the principal object 
to be attained is to find the maximum heating power of the sun 
under the most favourable circumstances. This is perhaps best, 
given bv the calorimeter depending on the rate of generation of 
steam,* which was designed by the writer, and used by him in Egypt 
in May 1882. The values of the solar constant obtained with 
the actinometer by observers of the highest standing differ greatly, 
and some of them are certainly exaggerated. What we want 
to know in physical geography is how much heat is quite certainly 
received from the sun by a given area of the earth’s surface ex- 
posed perpendicularly to its rays in a given time. With the 
writer’s steam calorimeter the highest rate actually realised was 
16°6 grms. of water converted into saturated steam of the same 
temperature per minute on an area of 1 square metre when the 
sun’s zenith distance was 20°. This is equivalent to 17°04 grms. 
steam generated by a vertical sun on the same area. The latent 
heat of steam at 100° C. is 535 gr.°C. per gramme, therefore the 
generation of 17-04 grms. of steam of 100° C. out of the same weight 
of water of the same temperature requires 9116 gr.°C. of heat; 
and this is the amount of heat which can with certainty be extracted, 
per minute, from a bundle of sun’s rays of 1 square metre sectional 
area at the sea level when the sun is at the zenith. The mechanical 
equivalent of heat is taken at 425 kilogramme-metres (kem.) of 
work per kilogramme-degree (kg.°C.) of heat, or per 1000 gr.°C. 
Converting 9116 gr.°C. at this rate we obtain as its equivalent in 
work 3875 kilogramme-metres (kgm.). This amount of work is 
done in one minute. One horse-power, or the rate at which the 
standard horse can work is taken at 4500 kgm. per minute ; therefore 
the working value of the sun’s rays at the sea level is at least 0°87 
horse-power per square metre for a vertical sun. The total area of the 
bundle of sun’s rays which is at all times being intercepted by the 
earth is the area contained by its great circle, and this is taken at 
* “On a Solar Calorimeter used in Egypt in 1882,’ by J. Y. Buchanan. Proceedings 
of the Cambridge Philosophical Society (1900), vol. xi. pp. 37, 74; and Nature (1901), vol. 
Ixiii, p. 548, j 
