Page:The Kinematics of Machinery.djvu/252

 230 KINEMATICS OF MACHINERY.

In the pumps, mills, and other machines used in the Middle Ages, we find crank or lever mechanisms very frequently employed, mechanisms, that is, in which turning-pairs are used for joints. If we examine these closely, and we have drawings which enable us to do this, we find in them a continual use of force-closure. The joints of cranks and connecting-rods are round bars enclosed in wide round eyes like the links of a common chain. Clumsily made collars, placed far apart, suffice to prevent excessive lateral motions. There Was play enough left to allow turning to take place, when it was wanted, about axes oblique to the axis of the shaft itself; thus in many cases where we would use a universal joint it was then unnecessary, the older form had fewer parts than the newer one.

In isolated survivals from former centuries, such as the ancient wine-presses which are even now to be found here and there in the Ehine and Moselle vallies and in Switzerland, we can still see force-closure in extraordinary completeness. A horizontal lever, generally made of oak,* working everywhere under force-closure, is used for transmitting the pressure ; in the oldest form its free end is loaded with a millstone. A screw (of wood) is then also applied, but this is not used for applying pressure, but for raising or lowering the loaded end of the lever. 41 In the Bhenish press, which must be considered the younger, the screw is used for' the application of pressure through the lever, the whole machine being an arrangement something like a screw-vice in which the pressure acts between the screw and the joint, and very near the latter, in- stead of upon one side of both.

The primitive iron hammers, which the pedestrian may still meet with in the busy little valleys of the mining districts of West- phalia, are also very remarkable. A little wooden water-wheel roughly made drives the tilt-hammer, and another the blowing- apparatus. Both machines are driven by force-closure, by means of projecting tappets or wipers, which act by downward pressure, the return motion in the latter being effected by a wooden spring- beam. There is scarcely a pair of elements in these machines, an inheritance preserved through so many centuries, which is not force-closed. By slow degrees our modern blowing-engine has developed itself from this primitive contrivance.


 * The "Kelter-baum " of so many songs.