LOCKS AS EATCHET-TEAINS. 467
lifts the click and moves the bolt, i.e., the key, is made separate from the lock itself. The key in many cases serves only to lift the click out of gear, a separate ratchet, connected with a handle, being used to move the rack. Complex forms are given to the key and the tumbler in order to render it impossible, or at least very diffi- cult, to move the bolt by any other key than that specially made for the lock.
The accompanying sketch of a Chubb-lock (Fig. 335), in which the different parts are marked with the letters used for corresponding parts in former figures, may make this matter somewhat clearer. The action of other safety locks, those of Bramah, Hobbs, Yale, &c., are so far the same. The art of lock-making indeed, which has been the parent of so many remarkable and ingenious inventions, has worked in its latest and most refined productions strictly in the spirit of kinematic science, it has followed its laws throughout with the greatest precision.
122. Brakes and Brake gear.
Brake drums or wheels are links of kinematic chains, made usually of the form ... | ... R, which serve to control or entirely to stop the motion of the links connected with them, by friction produced upon their surfaces. The blocks or band pressed upon the latter and the mechanisms connected with them form with the drum a complete brake. Brakes are applied both to pieces which move in straight, and to those which move in curved paths.
One fact about brakes which requires to be noticed is that the blocks, slipper or band, form with the drum or rod a pair of kinematic elements so long as the gear is in motion. If a drum be Used we have the pair (R), if the block acts on a bar or rail, the pair (P), and so on. Those brakes therefore which are employed completely to stop a motion, are used to prevent the action of a pair of elements, and this is done by uniting the partner elements in such a way that kinematically they may form a single piece only. "Under some circumstances brakes act in exactly the same way as click-gear ; there is, however, this difference between them
H H 2
