the prismatic sliding block than its guide; in plummer-blocks, spaces are left between the brasses, and in these themselves oil- grooves are frequently made, all of which cases are equivalent to removing the corresponding parts of the closed figure.
This procedure is so useful and is taken so much for granted in machine construction, where the portions of surface left are always of sufficient extent, that the question how far it can be carried scarcely presents itself to the designer. Where the working of a machine is accompanied by the transmission of forces of considerable magnitude, its designer furnishes it with large bearing-surfaces to prevent wear; but the question here is as to the absolute dimensions of the surfaces, and not as to
Fig. 49.
Fig. 50.
FIG, 51.
their distribution among the different bodies. If the forces are small, consideration as to wear scarcely enters into the question the extent to which the surfaces can be diminished is here limited by the consideration that what is left must be sufficient to ensure that the paired elements shall always occupy the intended posi- tions relatively to each other. The common cone-faced valve furnishes one among many illustrations of this. In the above three figures a simple valve, such as might be used to check the passage of water in a pipe, is shown in forms more and more removed from that of a solid cylinder. Such portions of the solid cylinder are always left to form the valve that in neither of the three cases can it be so moved that its axis shall not
