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STRETCHING   CAST   IRON,   ETC.                     429
It is natural to inquire as to the reason for the iron being stretched to such a large degree in these castings. The author's, hypothesis is that owing to the castings being filled with large cores containing both slim and thick cast and wrought core rods, as soon as the cores became heated they and all the rods expanded and, by outward pressure which they exerted, overcame the resistance of the outer body of the green sand mould; and while the metal was in a fluid state, instead of shrinking, as is generally the case with heavy castings, some of it would actually flow back and run out over the flow-off gates. This action continued until solidification took place; then stretching of the half molten or solidified iron came into play, expanding all sides of the green sand mould until the force of the expanding cores and their rods gave way to that of the outer mould's body of metal, and the casting attained that point of cooling, as shown in the experiments illustrated with the author's device, Fig. 89, in which it had cooled sufficiently to overcome the influence of the power most greatly exerted to stretch the iron, thereby exerting an expanding power at a time when the cooling iron was most susceptible to stretching, which, of course, varies according to the thickness of a casting, its rate of cooling, etc., to obtain a temperature from 1,600 degrees F. down to 1,200 degrees P., as cited on page 426, in the stretching tests with the apparatus above described.
The case of the pump which has been cited exhibits a form of power, proper to be classed as expansion and compression resistance to contraction. We still have another form, which I will call heat resistance, and which displays its power to stretch iron by reasonnts,    bin    of   aamples of which in every-day practice the writer will cite further on.cibly illustrates the great reason why the founder has to fear sulphur in fuel, high-sulphur iron, and to            j