In [Fig. 1484] is shown the form of surface plate designed by Sir Joseph Whitworth for plates to be rested upon their feet. The resting points of the plate are small projections shown at a, b, and c. The object of this arrangement of feet is to enable the plate to rest with as nearly as possible an equal degree of weight upon each foot, the three feet accommodating themselves to an uneven surface. It is obvious, however, that more of the weight will fall upon c than upon a or b, because c supports the whole weight at one end, while at the other end a and b divide the weight.
Fig. 1485.
[Fig. 1485] shows the form of plate designed by Professor Sweet.
Fig. 1486.
In [Fig. 1486] is shown a pair of angle surface plates resting upon a flat one. The angle plates may be used for a variety of purposes where it is necessary to true a surface standing at a true right angle to another.
The best methods of making surface plates are as follows:—