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178 are both alike, it is evident that they must be formed in pairs; that is, one right-hand and one left-hand, in order to attach them to the front of the head.

The pattern of the back, Fig. 254, is obtained by reproducing

the outline of the front elevation, Fig. 250, and adding laps, which are notched as shown.

80. Related Mathematics on Conductor Head.—Since all conductor heads are composites of the surfaces of many different solids, it is unpracticable to attempt to compute their exact surface area. The most accurate method of obtaining the cost of material entering into their manufacture is to lay out a full size