Chemical and Physical Notes 101 



(kg. C.) of heat, or per 1000 gr. C. Converting 91 16 gr. C. 

 at this rate we obtain as its equivalent in work 3875 kilo- 

 gramme-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 130 x io 12 square metres. There- 

 fore the working value of the sun to the earth is at least 

 113 x i o 12 horse-power. This figure depends on the amount 

 of steam actually generated in a particular instrument ; but 

 no instrument is perfect, therefore the above figure falls short 

 of the truth. One horse-power per square metre has been 

 taken as a probable work-value of the sun's vertical rays at 

 the level of the sea. This is equivalent to ro6 gr. C. per 

 square centimetre per minute. In accepting these values of 

 the solar heat constant at the sea-level we are assured that 

 we are not exaggerating. 



It is impossible to determine, or to estimate exactly, how 

 much of the sun's heat is absorbed in its passage through the 

 atmosphere. We have seen that Herschel estimates the 

 amount transmitted to be two-thirds of the amount which 

 arrives at the earth's orbit, leaving one-third to be absorbed. 

 The true amount absorbed is probably rather under than 

 over this figure. 



Taking I horse-power per square metre as the total work- 

 value of the sun's rays, and remembering that the mean 

 distance of the earth from the sun is 212 times the length of 

 the sun's radius, we find that the rays emitted by I square 

 metre of the sun's surface are spread over 2I2 2 , or in round 

 numbers, 45,000 square metres of the earth's surface. There- 

 fore, the probable work-value of I square metre of tJie suns 

 surface is at least 45,000 horse-power. 



It is useful to note that the sun's heating power at the 

 distance of the planet Mercury is 6 times, and at that of 

 Venus it is twice, its value at the earth's distance. 



