Page:The Kinematics of Machinery.djvu/526

 504 KIN EM A TICS OF MA CHINE R Y.

influence of the natural force acting upon it. The particular motion then occurring is made determinate by the chaining. In the place-changing machines this motion is used for the purpose of altering the position of the work-piece, in the form-changing machines with the object of altering its shape, the nature of both alterations being fixed by the form of the chain. Both results, the motion of a body in given paths and according to given laws, and the alteration (perhaps simultaneous) of its shape, are forms in which the machine has compelled the natural forces "to do work/'

A few general illustrations of this may be in place here.

In the common clock, in which we may suppose the chain increased in weight in the way before mentioned, if the chain be brought into such a position that it begins to uncoil itself from the barrel, every part of the mechanism will at once commence its characteristic motion. In clocks of the common construction motion ceases while the chain is being wound up, the process, that is, of bringing the chain into the " unstable " position just mentioned affects every part of the train. In clocks of a better class means are adopted for removing this defect, such as the use of a weighted lever arranged so as to come into action, and drive the mechanism during the operation of winding up. In other words a second kinematic chain is so placed in reference to the first, that when the latter is not acting on account of the winding process, the former is brought into an unstable position, into a position, that is, suited for driving the mechanism.

In the under-shot water-wheel we allow the pressure -organ, water, to act on the wheel as soon as the sluice is opened; the two members of the chain are so formed as to pair at once, and motion occurs through the action of gravity upon the water. In the turbine the water forms a screw pair S+S~ with the turbine wheel, it is caused to descend by gravitation, and therefore drives by its motion the element with which it has been paired.

The opening of the stop-valve of a steam-engine allows the column of steam to become a part of the chain (which we have seen to be a ratchet- train, 126), and to constrain it to perform that particular motion which its form permits. The indefinite length of the driving-link, the steam-column, is obtained by a physical process in the boiler. Similarly in hydraulic machines a ineteor- ologic process furnishes us continually with new portions of the