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STRETCHING   CAST   IRON,   ETC.                      419
the manner in which it is made and the casting is cooled, have much to do with the size of the casting, as compared with the pattern from which it was made. It is not the intention of the author to attempt to set forth fixed rules for the contraction of castings by the classification of the different kinds of work, as some have done, for this is not practical, but more to call attention to the principles involved and assist the engineer, founder, moulder and pattern-maker to best judge what contraction, if any, should be allowed for constructing patterns, to meet the various conditions in moulding, mixing of metals and cooling of castings. Not only has the experienced heavy-work founder found a great difference to exist in the contraction of the same kind of iron in different castings, but some will agree with the author in affirming that instead of allowing for contraction, the reverse conditions occasionally prevail and are elements frequently necessary to be considered in making patterns. It is nothing unusual for moulders and founders engaged in heavy or jobbing machinery to find their castings much larger than the patterns from which they were made, thus disclosing a condition in founding of which the light-work founder and " stove plater " would have no opportunity of obtaining any knowledge. Before the author discusses the qualities involved in stretching cast iron, which is an important part of this paper, he will consider those effecting a difference in thick and thin bodies cast under the same conditions or in the same flask with the same iron or '' gates'' and from which observing founders have learned that a heavy casting or parts will contract much less than a light one, where conditions permit of free contraction. HH, The upper half S was all y;reen sand3 per cent, of sulphur            I