J tieuiifit �ttt eritJu. PRACTICAL MECHANISM. BY JOSHUA ROSE. NU>lBKa XXIII. warping occurs depends upon the inequality of the thickness of the val ve in its various parts (it being always thicker in one part than in another), and upon the evenness with which it was allowed to cool after being cast. sides of the graver should not be ground upon the stone, the end only being ground, in the position shown in Fig. 69 , A being the grindstone, running in th e direction of the arrow B, the tool

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... and C , the graver. ' ROUGilING OUT. Our work, being countersunk, is now reatly tl) be turned down to nearly the required size all over. before any ORe part is made to the finished size. The reasons fordoing this are as follows: Upon the outside skin of all metal work a tension is produced. In wrought iron and other forged wo;k, this is caused by the working of the metal by the blacksmith, or, to a lesser degree, by the rolling mill, if the metal has been rolled. In iron, brass, or other castings, it is produced by unequal cooling after the metal has been cast, especi>\lly if the casting has been allowed to cool rapidly, as, for in stance, when the casting has been taken from the mold, as is commonly the case, wllile at a red heat. The effect of blows delivercd upon forged work by the blacksmith's tools, is not only grf\ater 1!1'011 the exterior than upon tlie interior of the