109 



the water ean be kept agitated by rotating the wbole apparatus. 

 Both Pouillet's and Hersehel's results agree very closely. Accord- 

 ing to the former, the amount of heat fallmg on each square centi- 

 meter of the earth, the sun being in the zenith, would in one 

 minute receive 1-7633 grammes of water through 1-6. Turning 

 this result into EngHsh measure, the heat received from the rays 

 of the vertical sun in one minute by a square of 4- 10th of an 

 inch side, are competent to raise 15-1 grains of 1° Fahr. ; a 

 result at first sight ridiculously small. 



5. Amount of Solak Heat, expressed (a) in terms of water 



HEATED. 



But at this rate the heat received by each square mile of the 

 earth's surface, were it all utilized in warming water, would in 

 one minute raise 750 tons from the freezing to the boiling point. 

 There being in the earth's section exposed to the solar rays some 

 49,000,000 square miles, the number of tons of water which 

 could be raised through the same range of temperature under 

 similar circumstances is simply enormous. More than this the 

 absorption exercised by our atmosphere is very great indeed, so 

 that according to Langley we should have to multiply the above 

 numbers by 1-7 ; in other words, nearly one-half the radiant heat 

 of the sun is stopped by our atmosphere. 



(b) In TERMS OF Ice melted. 



To regard this enormous quantity of heat from another point 

 of view. To melt IR. of ice, the temperature remaining at 32° 

 Fahr., 142-65 thermal units on the lb. Fahr. scale, which we 

 before adopted, would be required. Let it be supposed that on 

 the circle of the earth's orbit a mighty sphere of ice be construc- 

 ted, one inch in thickness. Since the distance of the sun from 

 the earth may be reckoned as 92,800,000 miles, twice this number 

 would represent the diameter of such a sphere. According to Sir 

 John Herschel it would take the sun's rays but two hours twelve 

 minutes, were their heat subtracted for no other purpose, to melt 

 this huge cyrstal globe. Another illustration from the same 

 source. Let a bridge of ice be constructed from the earth to the 

 moon, a distance of 240,000 miles. Make the diameter of its 

 cross section fifty miles. The volume of the resulting cyhnder 

 would be enormous. But were it possible to concentrate the 

 whole of the solar rays by any means but for one instant on this 

 ice- bridge, it would in that instant not only be melted but totally 

 driven into vapour. 



(c) In terms OF Work done. 



Work is said to be done when a force overcomes a resistance in 

 its own direction through any distance. Thus the hfting of a 

 mass against the attraction of the earth through a certain vertical 



