Fig. 2439.
Fig. 2440.
Suppose, for example, that No. 1 is concave in its length, and we fit No. 2 to it, as in [Fig. 2438], and then fit No. 3 to it as in [Fig. 2439], and when we come to put Nos. 2 and 3 together, as in [Fig. 2440], we find that they are out of true to twice the amount that No. 1 is, and that all the work that has been done to them to fit them to No. 1 has been thrown away, and possibly to make them worse instead of better. It becomes important therefore to select the most true plate for No. 1, and this we may do as follows:—
Fig. 2441.