196 THE ADDRESSES, LECTURES, ETC., OF 



both costly and restricted in its application. Although the force 

 is apparently obtained without expenditure, the intermittance of 

 the supply, the interest upon the outlay, the cost of maintenance, 

 and the tendency for such basins to silt up are drawbacks of such 

 serious nature that we may dismiss the question of the utilisation 

 of this source of natural energy from serious consideration. 



But what about the utilisation of the sources of energy depend- 

 ing upon the solar ray from day to day. It has been calculated 

 that the total calorific effect produced by solar rays upon the sur- 

 face of this globe would be sufficient in amount to evaporate 

 annually an ocean of boiling water covering the whole surface to 

 the depth of 14 feet, or to melt a stratum of ice of upwards of 

 100 feet in thickness. 



In order to produce the same calorific effect by means of a 

 theoretically perfect furnace, we must consider what quantity of 

 water is represented by a depth of 14 feet over the surface of the 

 earth. The earth's mean diameter is about 42,000,000 feet in 

 round numbers, and its mean circumference is 132,000,000 feet, 

 and the multiplication of these numbers gives its surface as 

 5,500,000,000,000,000 of square feet. If we multiply this 

 by the depth of 14 feet, and by the density of water, 62'4, 

 we obtain 77,000,000,000,000,000 of cubic feet, equal to nearly 

 5,000,000,000,000,000,000 of pounds of water. 



The heat which evaporates a pound of water, in a perfect boiler, 

 is about 1000 heat units, so that a pound of coal will evaporate 

 12 Ibs. of water, and a ton of coal about 27,000 Ibs. We 

 shall therefore require, taking only round numbers, about 

 180,000,000,000,000 of tons of coal per annum to perform the 

 effect of the sun's rays upon the earth's surface. This quantity is 

 about 660,000 times as great as the total quantity of coal raised 

 annually throughout the world. 



These figures prove that after all we are not so entirely depen- 

 dent upon the solar energy of former days, represented by coal, as 

 we have been apt to suppose ; but that, on the contrary, a vastly 

 superior supply of solar energy comes to us, year by year, by direct 

 radiation, which at the present time is actively employed in pro- 

 ducing summer and winter, the fertilising rain, the gentle winds, 

 the raging storm, and all those other natural effects which we 

 behold, but which we have not yet had occasion to utilise for our 



