Page:The Kinematics of Machinery.djvu/259

Rh I have already pointed out its direct parallelism with the primitive pulley-tackle43 (§ 40); but men have not yet ceased from admiring it and wondering at it, while its older counterpart is no longer thought worthy of special notice. The appearance of the "water-rod" (Fig. 177) which has been before described (§ 44) and its warm acceptance, show the same thing. I have already shown that this mechanism is simply the contrapositive of the endless band of Fig. 178, although it has followed it so late.

The ever-extending endeavour to do away with force-closure shows itself in interesting forms in some of the more refined outcomes of our modern machine construction, as for instance in mechanical lubricating-apparatus, and in various "safety" appliances, lock-nuts, nut-guards, split-keys, etc.

The mechanical lubricator replaces the force-closed supply of the fluid by the use of kinematic chains—sometimes of very complex form—which often solve the problem with great delicacy. The other arrangements mentioned substitute carefully made pair- or chain-closure for closure by friction or gravity.

In number of parts modern steam engines greatly exceed the older ones. In the engine of a common man-of-war, for instance, the parts required simply for securing the nuts increase the number of pieces in the machine by from 200 to 400.

Toothed wheels furnish us with another example.44 Although they have been known for thousands of years, their improvement to-day is still essentially in the direction of excluding force-closure, that especially which has remained with the "clearance" or "freedom" allowed between the surfaces of the teeth, and which