Page:The Kinematics of Machinery.djvu/260

 has often enough made itself disagreeably felt. In the Chinese winding mill (gin) and in the similar machine used by the Egyptians, and worked by water (the Sakkiah),45 there is a large amount of play left between the teeth, which were merely such rough blocks as rendered it possible for one wheel to drive the other. But we see that during the Middle Ages, and in the last few centuries, the freedom has been more and more reduced, as greater care has been taken to find the kinematic condition to be fulfilled by the form of the teeth-profiles, until we have now succeeded in reducing it to a very small fraction of the pitch. During the last century, the wheel and its teeth gradually came to be understood as forming together one whole, and the teeth-profiles were then looked at in a new light. I believe that in a few decades it will be the rule to employ spur-wheels working without any clearance between the teeth.

We have already noticed that the contest between pair- and force-closure continues in full activity in the department of prime-movers, where the question is one as to the mode of crossing the dead points of mechanisms. We saw that the double cylinder-engine was more and more taking the place of the single one. Even twenty years ago, capable although indiscriminating "practical" men said distinctly that the employment of the coupled engine in connection with mines was a mistake,—that the single machines, if only on account of their "simplicity," were greatly to be preferred; that men would soon change their minds, tire of these novelties, which were only fashions, and return to the old machine. To-day however, the double engine, in spite of the far greater number of its parts, is almost invariably used. In rolling-mills force-closure was, and is still, obtained by the use of colossal fly-wheels, which have too often, as is well known, been the cause of fatal accidents. As we have already mentioned in § 47, the latest forward step here, too, has been the substitution of the double for the single engine. Indeed it is very probable that another decade may see the coupled engine universally employed in spinning- and weaving-mills, machine-works, and manufactories of all kinds in which it