in Energy, the Poiver of Nature 33 



is potential, the latter kinetic. The raised weight or coiled 

 spring of a clock contains potential energy, which is gradu- 

 ally converted into the kinetic energy of moving wheels and 

 hands. The simple Pendulum consists of a heavy ball hung 

 by a thin cord. Its practical value depends on the fact that 

 if the length of the cord does not 

 change, the ball swings from one 

 side to the other in exactly the 

 same time through any small arc. 

 If the ball is pulled to one side to 

 A (Fig. 5), since the cord does not 

 stretch A is more distant from the 

 Earth's centre than is B, and when 

 let go its weight makes it swing 

 back toward B. At A the pendulum FIG. 5. Swing of a pendulum. 

 has a certain amount of potential &u. ^west point. 

 energy on account of its raised 



position, and as it falls it loses that potential energy, 

 gaining instead kinetic energy, so that it passes the point 

 B in the full swing of its active movement. The power 

 immediately begins to do work against gravity in raising the 

 ball to C, and the ball rises more and more slowly as its 

 kinetic energy is being used up until at C it comes to rest. 

 Here it possesses as much potential energy as it did at A, and 

 so swings back again. The swings are shorter and shorter 

 and finally it comes to rest only because the friction of the 

 air and of the cord on its point of attachment gradually 

 change all the energy into heat. 



55. Conservation of Energy is the term employed to 

 denote the fact that the total amount of energy in Nature, as 

 in the case of a frictionless pendulum in a perfect vacuum, 

 never varies ; that energy like matter can neither be created 

 nor destroyed. Many clever mechanicians have endeavoured 

 to find the Perpetttal Motion, by which a machine when 

 once wound up and set agoing would not only go on for ever, 

 but would do work as well. In January 1890 an advertise- 

 ment in the Times stated that the discovery had been 

 made, and the inventor wanted pecuniary help to com- 

 plete it. Knowledge of the laws of energy would have 



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