Page:The Kinematics of Machinery.djvu/528

 506 KINEMATICS OF MACHINERY.

of position along circular paths. In treating prime-movers as complete machines, then, popular usage has done not merely what is practically convenient but also what is in theory perfectly correct.

The question whether the lathe, planing-machine, spinning- machine, etc., are complete machines does not appear quite so easy to answer. We may suppose each of them to be arranged so as to be driven by a belt, for this purpose they only require to be fur- nished with suitable belt-pulleys. We may then certainly say, if the belt have always the tension necessary to prevent slipping, that they are complete. It is quite indifferent, so far as regards the chaining and the action of the machine, whether the belt be endless or not, whether it be moved by a weight or by muscular force (as in Berthelot's knotted belt Fig. 357,* or Borgnis' "flexible ladder" Fig. 358), or be driven from the shaft of an engine. In each case the belt is as truly the driver of the machine as the steam is of the steam-engine. Just as in that case it is indifferent whether the steam be received direct from the boiler or whether the engine be worked by the exhaust steam from another engine (as has been sometimes the case), so here it is indifferent by what means the belt be set in motion, it is and must in all cases be the driver of the machine.

In mining and tunnelling operations we often find prime-movers worked by air which has been compressed by a hydraulic air-pump, (as in the Mont-Ce'nis tunnel) or by a pump driven by steam. As prime-movers they are, however, complete, as long as the requisite quantity of driving air is supplied them. Their driver is the column of air in the pipes, and is itself set in motion by another prime-mover. This air-column between the two machines is, however, in precisely the same position as the belt between a steam-engine shaft and a lathe, loom, or any other machine driven from it. Our investigations have already shown us ( 44) that the cases are not merely analogous but essentially identical. But whether the driven machine be itself a prime- mover or be a machine directly employed in mechanical work is obviously beside the question. The machines considered

all being placed on the same shaft a. Borgnis, M6canique appliqute, Composition des Machines.
 * Three or more men work beside each other on as many ropes, the pulleys of