140 Proceedings of the Royal Physical Society. 



of ice there has been as much sun-heat employed as would 

 suffice to melt 5 lbs. of cast iron. 



Some other facts and figures showing the connection 

 between Solar Energy and the formation of ice may be 

 advantageously given here. The sun-heat at the Equator on 

 the day of the Equinox, on each square foot of the surface, 

 is equivalent to 1,780,477 foot-pounds. That is to say, from 

 each square foot of the ocean, then, and there, the Sun's heat 

 gets up steam sufficient to raise a ton weight to a height of 

 nearly eight hundred (794) feet. And the total heat received 

 per annum by the Earth from, the Sun is equivalent to 

 217,316,000,000,000 horse-power. In looking at a glacier or 

 an iceberg, one is apt to forget the enormous amount of solar 

 energy that has been expended in the formation of the mass 

 in question. 



When the fierce rays of the tropical Sun beat upon the 

 broad surface of the Atlantic, they do not all penetrate to 

 any great depth. The actinic rays, and also the light rays, 

 speed their way downwards through a vast depth of the 

 water, diminishing in energy but little as they go. But the 

 rays of greater amplitude, or those longer and slower 

 ethereal undulations to which heat is due, are nearly all 

 intercepted quite close to the surface. As a familiar illus- 

 tration of this fact, which is of much importance in the present 

 connection, reference may be made to the water-cell used in 

 the optical lantern. In this a thickness of only half an inch 

 of pure water will suffice to cut off nearly the whole of the 

 ]ieat-rays from such an illuminant as lime raised to incan- 

 descence by the oxyhydrogen flame, or even from that most 

 powerful of all artificial sources of heat and light, the electric 

 arc. The light passes through the water, but the heat is 

 stopped. So, in nature, nearly all the heat-rays of the Sun 

 are intercepted by the top layers of the sea- water, which, 

 accordingly, are the only parts directly warmed by the Sun. 

 It is, of course, only from the surface that evaporation takes 

 place, and it may be as well to remind the reader that 

 evaporation takes place at all temperatures — though most, of 

 course, at those which are highest, and from ice and snow as 

 well as from water. 



