To make a true surface-plate, it is necessary to plane up three plates, which we will term A, B, and C. First fit the faces of A and B together as nearly as possible with a smooth file (using a hardly perceptible coating of Venetian-red and common oil applied to A, to show by the marks where the faces touch). Then file C to fit A. Then try C with B; and if they mark all over the faces, they are true, and the surfaces may be finished by the scraper, trying them repeatedly as above. If, however, C and B should be found to fit on the edges only, it would demonstrate that A and B, though fitting, are not flat surfaces; but that A is hollow and B rounding. Having corrected these defects as nearly as possible, apply B to C, again repeating the correcting process until all three surfaces, applied interchangeably, mark evenly all over, when the surface of each will be practically true. It must, however, be borne in mind that, after rubbing the surfaces together to test, them, the middle of each plate will (from having received the most abrasion) show the marks the plainest, so that the plates will be more nearly true if the marks show a little the plainest at and near their edges, and less plain toward the centres. The back of each plate should be planed off, care being taken that it rests evenly upon the bench, so that neither plate shall deflect from its own weight, as it would do if its weight were not supported evenly upon the face resting upon the bench. The scraper should be used dry upon cast-iron, and kept moistened with water for steel, wrought-iron, and brass.