92 THE ADDRESSES, LECTURES, ETC., OP 



human mind would surely invent some other source of power 

 when coal should be exhausted, and that such a source would 

 probably be discovered in electricity. I heard such a suggestion 

 publicly made only a few weeks back at a meeting of the Inter- 

 national Jury at Vienna, and could not refrain from calling 

 attention to the fact that electricity is only another form of 

 energy, that could no more be created by man than heat could, 

 and involved the same recourse to our accumulated stores. 



If our stores of coal were to ebb, we should have recourse, no 

 doubt, to the force radiating from the sun from year to year, and 

 from day to day ; and it may be as well for us to consider what 

 is the extent of that force, and what are our means of gathering 

 and applying it. 



Growth of Plants. We have, then, in the first place the 

 accumulation of solar energy upon our earth's surface by the 

 decomposition of carbonic acid in plants, a source which we know 

 by experience suffices for the human requirements in thinly 

 populated countries, where industry has taken only a slight 

 development. Wherever population accumulates, however, the 

 wood of the forest no longer suffices even for domestic require- 

 ments, and mineral fuel has to be transported from great distances. 



Water Power. The sun's rays produce, however, other effects 

 besides vegetation, and amongst these, that of evaporation is the 

 most important as a source of available power. By the solar 

 rays, an amount of heat is imparted to our earth that would 

 evaporate yearly a layer of water fourteen feet deep. A con- 

 siderable proportion of this heat is actually expended in evapo- 

 rating sea water, producing steam or vapour, which falls back 

 upon the entire surface of both land and sea in the form of rain. 

 The portion which falls upon the elevated land flows back towards 

 the sea in the form of rivers, and in its descent its weight may be 

 utilised to give motion to machinery. Water power, therefore, is 

 also the result of solar energy, and an elevated lake may indeed 

 be looked upon as fuel, in the sense of its being a weight lifted 

 above the sea level through its prior expansion into steam. 



This source of power has also been largely resorted to, and 

 might be utilised to a still greater extent in mountainous 

 countries ; but it naturally so happens that the great centres of 

 industry are in the plains, where the means of transport are easy, 



