Page:Miscellaneous Papers on Mechanical Subjects.djvu/20

 12 by the scraping instrument, and the operation is frequently repeated. As the work advances, a smaller quantity of colouring matter is used, till at last, a few particles spread out by the finger suffice for the purpose, forming a thin film over the brightness of the plate. A true surface is thus rendered a test of the greatest nicety, whereby the smallest error may be detected. At this stage of the process, the two surfaces must be well rubbed together, that a full impression may be made by the colour. The higher points on the rising surface become clouded over, while the other parts are left more or less in shade. The dappled appearance thus produced, shows to the eye of the mechanic, the precise condition of the new surface in every part, and enables him to proceed with confidence in bringing it to correspondence with the original. Before this can be accomplished, however, the scraping instrument must be employed, the file not having the precision or nicety requisite to finish the operation. Experience will be a sufficient guide when to exchange the one for the other. It will be found, that when the parts to be operated upon have become to any considerable extent subdivided, scraping is much the more expeditious method. The instruments should be made of the best steel, and carefully sharpened to a fine edge on a