OBJECTS.] INTRODUCTORY. 45 



thrust violently backwards by a strong man. The 

 boat will go stern foremost rapidly, at first, but every 

 stroke of the boy's oar at the stern will retard its back- 

 ward motion ; until, at length, the stock of momentum 

 conferred "upon it by the man's thrust will be com- 

 pletely exhausted in working against the boy, and the 

 boat, after a momentary rest, will resume its onward 

 course. The distance to which the boat will be pro- 

 pelled backwards will evidently depend upon the 

 amount of muscular power which the man, as it were, 

 suddenly capitalizes in the boat, and which the boat 

 then slowly pays out. 



We call people who possess much muscular or 

 other power energetic ; and we estimate thsir energy 

 by the obstacles they overcome, or, in other words, 

 by the work they do. In the present illustration 

 the man's energy would be measured by the distance 

 to which the boat was propelled before it stopped. 



It is easy to transfer this conception of energy, as 

 the power of doing work, to inanimate things ; and 

 thus, when a body in motion overcomes any kind of 

 obstacles in its way, parting with its momentum and 

 more or less coming to rest in the process, we say that 

 it has energy and that it does work. 



The energy of moving water is thus measured by 

 the intensity of the opposing forces which it can 

 overcome multiplied into the distance which it can 

 travel before that energy is exhausted ; that is to say, 

 by the work it does before it is itself reduced to a state 

 of rest. In the case under consideration, the energy 

 by which gravity is overcome, for a greater or less time, 

 depends upon the velocity of the stream ; and this again 

 depends upon the height of the water in the vat 



