Page:On the economy of machinery and manufactures - Babbage - 1846.djvu/121

Rh costly of execution by the hand; but they become easy and comparatively cheap, when made by pressing the soft material out of which they are formed into a hard mould. The care and skill bestowed on the preparation of that mould are repaid by the multitude it produces. In many of the works of the china manufactory, one part only of the article is moulded; the upper surface of the plate, for example, whilst the under side is figured by the lathe. In some instances, the handle, or only a few ornaments, are moulded, and the body of the work is turned.

(115.) Glass Seals.The process of engraving upon gems requires considerable time and skill. The seals thus produced can therefore never become common. Imitations, however, have been made of various degrees of resemblance. The colour which is given to glass is, perhaps, the most successful part of the imitation. A small cylindrical rod of coloured glass is heated in the flame of a blow pipe, until the extremity becomes soft. The operator then pinches it between the ends of a pair of nippers, which are formed of brass, and on one side of which the device intended for the seal has been carved in relief. When the mould has been well finished and care is taken in heating the glass properly, the seals thus produced are not bad imitations; and by this system of copying they are so multiplied, that the more ordinary kinds are sold at Birmingham for three pence a dozen.

(116.) Square Glass Bottles.The round forms which are usually given to vessels of glass are readily produced by the expansion of the air with which they are blown. It is, however, necessary in