460 KIN EM A TIGS OF MA CHINEE Y.
upon the nature of particular classes of constructive elements. We must therefore somewhat overstep the usual limits assigned to the constructive elements themselves.
We have already pointed out that click-trains are employed both for direct and reversed motion. We shall first examine shortly the nature of the reversed motion of the click-piece and the corresponding relative motions of the pawl or click.
If the click-piece a be a wheel, its reversed rotation gives to the pawl C.,.\\...Z also a motion of rotation, the conditions of which depend upon the particular form used for the back of the teeth of a. This may be considered, in the most general case, as a portion of the curved profile of a disc or cam a, Fig. 326. The motion given by this profile to the pawl will be an oscillation about the axis of the pair connecting it to the frame c. If the FlG 396 radius of the pawl be made infinite it be-
comes a block b (Fig. 327), which, with
suitable force-closure, is caused to slide to and fro in a straight line by the cam a. Here we have, as before, a chain of three links ; its formula is
a b c
C+ ... || ... 0,j... ...P+P- ... JL ... C-.
By placing this chain upon the link c we obtain a very numerous series of mechanisms, which we may call slider-cam trains. There are various ways in which the force-closure of Z may be replaced by pair-closure. One of these is the addition to the end of the block or pawl w r hich is paired with the cam of a cylindric pin an element, that is, of which the profile is a curve equidistant to the end-point, and the pairing of this pin with a groove in the cam a, formed by drawing equidistants to the original profile, Fig. 328 (cf. 35). The force-closure of the pawl or block b is therefore not an essential feature in the cam train, but only a separable and accidental property of it. We were therefore fully justified in the omission from our formula, in the last section, of the sign for force -closure. We must not go further into this question here ; it is one which must receive
