1 8 Elementary Biology. 



flow. Tidal motion is a source of energy little used, 

 although in many cases there seems to be no feasible objec- 

 tion to its employment. 



2. Ordinary water-power and falling bodies in gene- 

 ral. This source of energy has been employed from very 

 ancient times. The possession of potential energy by water 

 depends on its being at an elevation, and its transformation 

 into kinetic energy on its being allowed to fall or flow to a 

 lower level. An undulating or mountainous country will be, 

 therefore, a country with the greatest available water-power, 

 not only on account of the many differences of level, but 

 also on account of the greater rainfall in a mountainous 

 country, and consequent greater absolute supply of water. 

 The energy available by the fall of water from a height is of 

 the same nature as that available by the fall of solids, for 

 example, various forms of hammer, clock-weights, c. 



3. "Wind currents, Universally employed as a motive 

 power in navigation, mills, c., but a source of energy not 

 always to be depended upon, and often adverse to the 

 furtherance of the object for which it is wanted. 



4. Water currents. Ocean currents are, under certain 

 circumstances, useful as an assistant motive power ; whilst 

 river currents are not infrequently employed for the carriage 

 of material to the sea, e.g. the carriage of wood on the St. 

 Lawrence. 



5. Hotsprings, volcanoes, and earthquakes. These 

 sources, although prodigious in amount, in addition to being 

 only locally developed, and in regions where their energy 

 is, as a rule, not wanted, are so utterly unreliable, that they 

 may be left out of account entirely, at least until a method 

 of transforming their energy, by storage or otherwise, into 

 reliable forms be discovered. 



