TOO Chemical and Physical Notes 



at a height of 1850 metres above the sea. This calculation 

 has been carried out 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 actino- 

 metric 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 circum- 

 stances. This is perhaps best given by the calorimeter 

 depending on the rate of generation of steam 1 , 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 exposed perpendicularly to its rays in 

 a given time. With the writer's steam calorimeter the highest 

 rate actually realised was i6'6 grms. of water converted into 

 saturated steam of the same temperature per minute on an 

 area of I 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 i 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 (kgm.) of work per kilogramme-degree 



1 'On a Solar Calorimeter used in Egypt in 1882,' by J. Y. Buchanan. 

 Proceedings of the Cambridge Philosophical Society (1901), vol. xi. pp. 37-74; 

 and Nature (1901), vol. Ixiii. p. 548. (See below, p. 337.) 



