RECENT WONDERS OF ELECTRICITY. 671 



that lie has sent something out of his body into the ball which hurtles 

 through the air past the goal, and the game is won. In all these illus- 

 trations something is done which results in fatigue, work is performed 

 and energy is lost ; in fact, work done means euergy applied, and 

 energy applied means work done. As mental energy is our capacity 

 for learning lessons, for going through examinations, and that kind 

 of thing, so the energy of the kind I speak of is the capacity for doing 

 absolute physical work. The generality of this energy is immense. 

 It is a difficult thing to grasp the fact that there is something in exist- 

 ence that we can not feel, that we can not touch, and that we can not 

 see, but which gives us all the force and all the power we possess. 

 The earth as it moves around the sun and the earth as it daily rotates 

 upon its own axis are instances of the existence of this energy ; in 

 fact, every kind of change produced in the condition of matter, whether 

 it be in its physical state or in its position with reference to fixed 

 objects, means energy gained or energy lost. The energy gained or 

 lost is the quantity of work done on a body, or done by that body ; 

 so I want you to grasp, if you possibly can, the fact that every case 

 of motion on this earth even the sound of my voice, as I now speak 

 to you is due to the exercise of this particular energy. Lecturing, 

 for instance, is a considerable exercise of energy. A man can lecture 

 pretty well for an hour ; he can lecture not quite so well for two hours, 

 but the best lecturer in the world can scarcely keep at it for three hours 

 without losing all his energy, and, like the crew in the boat, being 

 thoroughly pumped out. This energy or work done, as I have said 

 is measurable. The gardener who moves his ton of gravel three 

 feet high has done an amount of work that any boy can calculate who 

 will reduce one ton to pounds, and multiply that sum by three. He 

 will then have obtained 6,720 what we call foot-pounds. One pound 

 raised one foot high is the common and ordinaiy unit of work ; 33,000 

 pounds moved one foot high is called a horse-power, and the horse- 

 power is the mode by which the power of steam-engines is measured. 

 One steam-engine is known from another by its being able to exercise 

 ten, twenty, or fifty horse-power ; and so, whether it is merely the 

 lifting of gravel or a small tube, such as I have here throwing a 

 cricket-ball, moving a boat, or working a steam-engine, it can all be 

 measured by this simple unit of foot-pounds. Two other terms are 

 necessary in order to make you comprehend what energy is. Energy 

 is found in two conditions called " potential " from its position, and 

 "kinetic" from its motion. The potential form of energy is that 

 which exists in the form of a wound-up watch-spring : you wind up 

 the spring of your watch, and by doing so you pass something from 

 your body into it you transfer energy from your own body into the 

 watch, the spring is wound up, and the watch thereby can be kept go- 

 ing for twenty-four hours or more from the storage of energy imparted 

 to it. A clock, again, will go for eight or fourteen days by your wind- 



