relationship between force and motion, be distinctly separated, for we have always one class of machines in which the forces have chiefly to be considered, while in another the motions are more important. The ways in which men's ideas have developed in regard to the two questions have also been quite different.
Man has always had before him in nature moving forces, but so far as these were beyond himself they were in the beginning unrecognized and unknown; he had first to learn how to distinguish and separate them from the multitude of accompanying phenomena, —to discover them. Thus the development of the machine on its dynamic side has been closely connected with men's knowledge of nature, with what grew later on into natural science, with which it became more and more closely connected. In inventing the steam engine Papin was as much a physicist as a mechanician, and the same may be said of Watt when his searching genius grasped the subject. And so to-day the most exact resources of mathematical and experimental physics are employed in the discovery and accurate scientific investigation of the various sources of energy.
In the same way, but in even greater variety, man has always had motions before him in nature; but these have always been either kosmically free motions or such as were directed by some animated intelligence,—never, or extremely seldom, those closely limited and regularly interdependent motions which we find in the machine. This constrainment is obtained only as the result of thought, man has had to create it through an intellectual act, in other words to invent it. Discovery on the one side, invention on the other; in this antithesis we have the difference between the dynamic and the kinematic development of the machine. The discovery of each new source of energy leaves it to invention to supply means for utilizing it. The discovery of the dynamic properties of steam, for instance, may rather be said to have rendered progress possible than to have been itself a step forward. It called forth the most energetic activity of thought, the most careful reflection and study, in order to create, by invention, means by which the new sources of power might be utilized. Invention remained unceasingly active in its endeavours to extend the applications of these, its consciousness of its own object becoming gradually more and more distinct until to-day it is in part
