CBOSSHEADS AND GUIDES.
455
block c of Fig. 318. As a matter of history the crosshead has passed through an unusually large number of changes of form, which show of what careful study it has been the subject. The
FIG. 318.
production of an exact rectilinear motion in a given mechanism a matter which at first sight appears so simple is a problem which for a long period remained without a practical solution.*
Fio. 319.
We have now seen that the crank, the connecting-rod, the cross- head, the guide bars (including with them the crank shaft bearing), and the lever are in fact all the links of the crank trains
H9. Click Wheels t and Gear.
The exact treatment of click- or catch-gear leads to very complex and many-sided problems. We cannot attempt to treat these
- Cf. end of 3.
t For the purposes of Prof. Reuleaux's work it has been necessary to distinguish between two classes of ratchet gear, that, namely, in which the pawl or click acts merely to prevent motion, and that in which it is used to drive the wheel or rack. I propose to call the first class click-gear, and to use the common name of ratchet - gear only for the second and more important division.
