226 KINEMATICS OF MACHINERY.
stored, the sensible force of the muscles is made latent in it, and it is this latent energy stored in the elastic bow which actually propels the arrow. In the ballista and the catapult this principle receives still more extended application, for in them kinematic means are employed to store the muscular energy of many men, so as to em- ploy it concentrated with correspondingly increased effect. Later on the same principle extends itself to primary forces, and it is to-day more used than ever, from the tiny watch-work or the spring of a gun-lock through innumerable mechanisms up to the Armstrong accumulator, or the air-vessels of the Mont Cenis borers.
The discovery of the motive-force of steam occurred late, long after that of some explosive materials was known ; in each we use simply the latent forces which nature has distributed upon the earth in such enormous profusion in her decomposable materials. This discovery gave to men a source of energy of which the importance was at first not seen, but which has raised the machine into a power in nature sufficient to .have made an entire revolution in the life of the human race.
49.
The development of the Machine from a Kinematic point of view.
The question now arises : what is the special kinematic meaning or nature of the changes by which the machine has advanced to its present degree of completeness ? What has been near to, and what far from the spirit of invention, if we indi- cate by that name the recognition, becoming clearer and more distinct gradually from the remotest times, of the mechanical in the machine ? I believe that the answer to this question is : the line of progress is indicated in the manner of using force- closure, or more particularly, in the substitution of pair- closure, arid the closure of the kinematic chains obtained by it, for force-closure.
The notion which the gradually expanding mind of the primi- tive man first connected more or less dimly with his machine was the constrainment of certain motions in lifeless bodies for his own purposes. The forces necessary for these motions he sought in
