RECEPTOR, COMMUNICATOR AND TOOL. 487
by facts or can remain useful to us. I alluded in the Introduction to the widely-diffused conception of the nature of complete machines which was specially supported by Poncelet's authority and which has taken such firm root in the French mechanical instruction. This conception is that the complete machine is in general a com- bination of three parts or groups of parts,
1. The receiver of energy, or receptor,*
2. The parts transmitting motion, or communicator,
3. The working parts, or tool.
The part or group of parts constituting the receptor is generally understood to be that upon which the natural force driving the machine acts directly ; the tool is the part by means of which the energy received by the machine is directly expended in producing the required change in or in connection with the body to be worked on. As the motions of these two sets of parts are seldom identical, the second group of parts is required to transmit motion from one to the other. The whole conception has something so direct and simple, we might almost say natural, about it as to give a most favourable impression. Poncelet himself spoke of it as well- established as a matter of which the logical completeness was entirely convincmg.f In saying this, too, he rather put in a few words what was previously known, and more or less distinctly recog- nised, than expressed something entirely new to his time, and now the idea has become to some extent a part of the very foundation on which the study of machinery, in France at least, is based. There is indeed much to be said for its directness, and the ease with which it can be grasped. Its division of its subject into three parts, a begin- ning, middle and end two principal parts connected by a third prepossesses us in its favour by presenting a certain analogy with
- A few English authors have mentioned this classification, but none except
Moseley have, I think, ever used it to any considerable extent. Both words and ideas will be somewhat unfamiliar to English readers, while Prof. Reuleaux's views will not run counter to any preconceived ideas here as they have done on the continent. The controversial part of the following sections would therefore have been unnecessary had they been originally written in English ; the conclusions arrived at are, how- ever, none the less valuable.
t TraiU de M&canique Inditstrielle, Pt. Ill, 11. "La science des machines, ainsi envisagee, se compose done de la science des outils,de la science des moteurs, et de la science des communicateurs ou modificateurs du mouvement . . ."
