672 ANNUAL REPORT OF THE Off. DoC. 



that woi-k does not do itself. When we go into a mill or factory 

 and see some great machine at work, we know that the machine itself 

 does not produce the power which we see exerted. If we examine 

 more closely, we find perhaps a belt or gear connecting this machine 

 with a shaft, and following this up still further, we find perhaps 

 another belt until we finally come to the water wheel and discover 

 that it is the pressure or the weight of the water in the reservoir be- 

 hind the mill which is the source of the power exerted. When we 

 stand beside the track and see a great locomotive go thundering by, 

 we know that it is not the iron and steel of the machine which fur- 

 nishes the power, but the steam in the cylinders, and that this in its 

 turn owes its existence lo the burning of the fuel under the boiler. 

 When we see the trolley car go whizzing along the streets, we trace 

 up mentally a similar but more complex chain and find, as in the loco- 

 motive, the ultimate source of the power exerted to be the fuel burn- 

 ing under the boilers in the power house. In all these cases, and 

 in all instances where power is exerted, something does work. This 

 something we call energy and we, therefore, define energy as being 

 the power of doing loorJc and speak of the energy of the falling water 

 or of the burning coal. 



Let us now take another step: Most of us have probably watched 



a pile driver at work. The heavy weight is drawn to the top of the 



machine and let go; as it falls, it gradually acquires an increasing 



velocity, and at the moment when it strikes the head of the pile it 



is able, by virtue of this velocity, to do work upon the pile. That is 



to say, the falling weight of the pile driver contains energy. After 



the weight has fallen, it is again drawn up to the top of the machine; 



in so doing, work is done upon the weight either by muscular energy 



or, more probably, by the energy of coal burning under a steam 



boiler. When the weight has been drawn up and fastened at the 



top of the frame, it contains the potentiality of doing work. So long 



as it remains there, it does not actually do any work, but that we 



have to do to get work out of it is to allow it to fall; in this sense, 



then, the weight at the lop of the pile driver has the power of doing 



work and this is equivalent to saying that it contains energy. It 



is plainly, however, a dillerent sort of energy for that which it had 



at the moment of striking the pile. We call it j)otential energy, 



meaning by this that it contains the possibility or potentiality of 



work. The energy which the weight contains when it strikes the 



top of the pile, on the other hand, we call actual energy or energy of 



motion or hhutic energy. A swinging pendulum affords a very good 



illustration of these two foi-ms of energy and the conversion of the 



one into the other; when lifted to one side, it contains a certain 



amount of potential energy; when it reaches the middle of the swing, 



that energy has been converted into kinetic energy; as it rises this 



