264: CELESTIAL DYNAMICS. 



III. MEASURE OF THE SUN'S HEAT. 



THE actinometer is an instrument invented by Sir John 

 Herschel for the purpose of measuring the heating effiect 

 produced by the sun's rays. It is essentially a thermometer 

 with a large cylindrical bulb filled with a blue liquid, which 

 is acted upon by the sun's rays, and the expansion of which 

 is measured by a graduated scale. 



From observations made with this instrument, Sir John 

 Herschel calculates the amount of heat received from the sun 

 to be sufficient to melt annually at the surface of the globe a 

 crust of ice 29'2 metres in thickness. 



Pouillet has recently shown by some careful experiments 

 with the lens pyrheliometer, an instrument invented by him- 

 self, that every square centimetre of the surface of our globe 

 receives, on an average, in one minute an amount of solar 

 heat which wo,uld raise the temperature of one gramme of 

 water 0'4408. Not much more than one-half of this quan- 

 tity of heat, however, reaches the solid surface of our globe, 

 since a considerable portion of it is absorbed by our atmo- 

 sphere. The layer of ice which, according to Pouillet, could 

 be melted by the solar heat which yearly reaches our globe 

 would have a thickness of 30-89 metres. 



A square metre of our earth's surface receives, therefore, 

 according to Pouillet's results, which we shall adopt in the 

 following pages, on an average in one minute 4'408 units of 

 heat. The whole surface of the earth is = 9,260,500 geo- 

 graphical square miles* ; consequently the earth receives in 

 one minute 2247 billions of units of heat from the sun. 



In order to obtain smaller numbers, we shall call the 

 quantity of heat necessary to raise a cubic mile of water 1 



* The geographical mile = 7420 metres, and one English mile = 1608 

 metres. 



