Fig. 2442.
If we have a straight-edge that is known to be true, we may lay it on the face of a plate and move it laterally from each end alternately, and if it swings from the centre the plate face is rounding, while if it shuffles across moving first at one end and then at the other the face is hollow; but if it glides as it were across, the surface is nearer true. The straight-edge must not be pressed to the plate, but merely touched laterally to make it move laterally, for if we take a true straight-edge and press it vertically to a true surface while moving it, it will show the marks of contact the most plainly immediately beneath the parts where it is pressed. Selecting by this means the two plates that appear to be the most true we proceed to test them further as follows: We give to one of them which we will call No. 1 a light coat of red marking, and placing it upon the other or No. 2, we move it about in all directions and then take the two apart to examine the bearing marks. Suppose then that No. 1 shows the bearing marks to be at the shaded places, a and b, in [Fig. 2441], while the bearing marks on No. 2 are as at the shaded parts a and b in [Fig. 2442], the two ends a having been placed together; then we know that b is a high spot on No. 1, and a a high spot on No. 1 for the following reasons. The marks at a, No. 2, have been made by the marking at a on No. 1, and will extend across No. 2, a distance depending upon how much No. 1 has moved across No. 2, for if corner a of No. 1 had only moved half-way across No. 2, it could only have marked half-way across it. Similarly spot b on No. 1 has marked spot b in No. 2, because it has been moved all the way across, it being evident that the marking on b, No. 1, can only mark plate 2 as far across its width as it is moved across it. From this it follows that the higher or more prominent a spot is the less will be the area of the bearing mark at that spot.
Fig. 2443.
Fig. 2444.