Page:The Kinematics of Machinery.djvu/61

. As a whole, therefore, the specialised study of machines (specielle Maschinenlehre) considers both.

The third science is that of Machine-design. It also has been freed by Redtenbacher from its incorrect treatment under Applied Mechanics, and placed by him on an independent footing. Its province is to teach how to give to the bodies constituting the machine the mentioned in our definition. In order to determine this property fully it must be considered in reference not only to sensible but also to latent forces.

The first it accepts as found by the aid of the Science just examined, in the shape for example of the steam pressure upon a piston, the water pressure in a turbine, and so on; these determine the strength of the bodies. The latter, the latent forces, carry the force-action from body to body,—e.g. from piston-rod to connecting-rod, from spur-wheel to spur-wheel, and so on; and cause therefore necessarily friction and wear. The problems of machine-design extend in both the directions thus pointed out. In solving these problems in such a way as to conform to the conditions of each special case, machine-design forms itself into a really  science. Its twofold nature, as concerning itself both with sensible and with latent forces, which hitherto has been recognised in fact without being known to theory, I wish to raise into the position of a leading principle; its reality has been clearly proved from the general development of fundamental propositions.

Now, lastly, our definition covers a fourth characteristic of the machine which has not been a leading idea in either of the three studies we have considered. This is the arrangement of the means for insuring that only  shall occur in the machine. So far certainly as the motions are conditioned by forces, and are regarded solely in connection with force-actions, they have been considered in studying the theory of machines in the way already described. But that study simply takes the motions looked at as  as given. Hence another series of investigations remain, their subject being the nature of the mutual dependence of the changes of position of parts of the