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238                        METALLURGY   OF   CAST   IRON.
pin O was removed it was set 011 a level clean surface, a wedge T passed along until it was stoppd by a pin, as at U. The distance the wedge passed under the pin U was measured by a micrometer at V, Fig. 55. The depth of such holes could also be measured by filling them with water and measuring it with a small graduate, shown at W, Fig. 55. The structure, column No. 8, is given merely to denote distinctions as made by the eye in judging the relative size of the crystals or grain of the fracture. For example, No. 2 stands for what would be expected of the grain in a piece of true No. 2 iron, and so on up with closer iron in the higher numbers.
The iron was melted in the twin shaft cupola seen in Fig. 56, the operation of which is explained in pages 325 to 327. The use of such a cupola is the most reliable one for making comparative tests, which involve delicate observations and affords a remarkably uniform conditions of fuel, blast, heat, etc., necessary to discover, the true effects of changes in the elements composing cast iron. The fact that the tests in Table 35 were obtained with the use of the cupola, Fig. 56, gives the writer greater confidence in the results shown than he could place in any others obtained in the ordinary way of making separate comparative tests, that is, having one heat taken off one day and another some other day, with the differences in fuels, blast, and heat conditions that usually exist in  making heats in ordinary cupolas.
In charging the cupola, Fig. 56, small pieces from the same pig were placed in each compartment and the ferro-manganese placed on one side only. For the second and fifth heats, shown in Table 35, two "	.401.	1-375	.040	Mottled