430 METALLURGY OF CAST IRON"-
of the carbon being more completely transformed to graphite tinder slow cooling. An example of this is an experiment which was made by a N"ew York City founder some years ago.
The feat achieved by the founder was that of casting a balance wheel of about 18 inches diameter, having a rirn about two inches thick, with four to six arms only about one-quarter inch thick. The wheel was on exhibition for some time and the wonder of founders was how it held together. The author was informed that the secret lay in a heating device, so arranged as to keep the arms at a high temperature and to preserve the temperature close to that of the rim, as the latter was cooled off. The author would say that the feat was not achieved wholly by reason of extended heat, evolving greater graphite carbon in the arms. The element of stretching also assisted while keeping the arms hot, thus permitting the pulling power of the rim to extend them.
When we consider the difference that naturally exists in the contraction of light and heavy bodies, so clearly displayed in the test cited, pages 390 and 420, of a four by nine and one-half by two bar, it cannot but be evident that had the above wheel been left to cool olT naturally, the arms would have pulled away from the rim. This founder's achievement involves a lesson not to be forgotten by any interested in the founding or designing of machinery.
The ignorance which prevails on the question of contraction is very often astonishing. It is only the fact that east iron will stretch that saves many from having their ignorance on this subject exposed. There are many castings made that would not hold to-er 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