414 METALLURGY OF CAST IRON.
It is to be remembered that the tests of iron shown in Table 86 do not include an iron as soft as is necessary for stove plate or very light castings, and because such grades of iron are softer than any shown in Table 86 they would possess less shrinkage. The tests exhibited by Table 86 demonstrate positively that metal will shrink and cause trouble by leaving holes in the interior of castings, and also that the greatest shrinkage exists in the harder grades of iron.
The relation that contraction maintains to shrinkage, with the same metals (see page 386), was another point which the author thought well to obtain knowledge of while conducting the experiments on shrinkage. In order to test this factor the author devised the appliance seen in Fig. 86, and which permitted casting bars seen at the left of this figure in a sand and chill mould, to test, together with other qualities, the difference in contracting that would be caused by rapid and slow cooling of the same metal. By Table 86 we find that tests Nos. i and 6 give us the mean of .127 greater contraction for the fast cooled bars than for the slow cooled ones, each of the same cross section "and length, patterns for which are seen at the left of Fig. 86. The greatest difference in Table 86 is . 225 and the smallest .030. It is to be remembered that the respective tests seen in Table 86 were cast in their order with the same gate and hand ladle of iron. The cause of such a difference in the contraction of two bars is, as will be seen by Fig. 86 at N, that one is cast in a chill mold and the other in sand, P being the space for molding the sand bar. A study of the difference in contraction which the rate of cooling can cause by the device seen at Fig. 86 is instructive in more ^