Solar Radiation 393 



and at 1 1 h. 23 m. As the collecting area of the reflector 

 was 904 square centimetres, this corresponds to i6'6 c.c. 

 distilled per minute per square metre. If we apply a cor- 

 rection for 20 zenith distance it becomes 17*04 c.c. The 

 evaporation of 17*04 grammes of water at iooC. requires 

 9116 gr. C. of heat, so that the heat actually collected 

 and used in making steam was at the rate of 9ii6gr.C. 

 per square metre or 0-9116 gr. C. per square centimetre 

 per minute. Converting 9ii6gr.C. into work at the rate 

 of 0-425 kilogramme-metres per gramme-degree, we obtained 

 as the realised working value 3875 kilogramme-metres per 

 minute or 0*87 horse-power per square metre. The reflector 

 consists of one mirror inclined at an angle of 45 to the 

 axis of the instrument. This mirror throws all the reflected 

 rays normally on the surface of the axial border. The 

 larger mirror outside and the smaller mirror inside of this 

 one throw their reflected rays inclined at small angles to the 

 normal. Taking all the reflected rays together their mean 

 normal component is 94 per cent, of the total reflected rays. 

 It is therefore legitimate to increase the above numbers in the 

 proportion of 94 : 100, giving 0-93 horse-power or 9700 gr.C. 

 per square metre per minute. Allowing 7 per cent, for loss, 

 we have in round numbers for the work-value of the sun's 

 vertical rays on the surface of the earth at or near the sea- 

 level i horse-power per square metre ; the equivalent of this 

 in heat is 10,590 gr. C. per square metre per minute, or 

 1*059 gr. C. taking the square centimetre as unit of area. 



Mr Michie Smith informs the writer that the highest rate 

 which he has observed at Kodaikanal is 1754 c.c. distilled 

 per minute at a height of 7000 feet above the sea. This 

 result taken together with 1-501 c.c., the maximum amount 

 distilled per minute on the banks of the Nile, gives a measure 

 of the absorptive power of the corresponding layer of the 

 atmosphere in tropical regions. 



Considering Crova's summer values as determined at 

 Montpelier and Vallot's (1891) values for Mont Blanc and 

 Chamonix, we find that the rate at which the surface of the 

 earth at the level of the sea receives heat under the most 



