When a surface has been reduced to approximate flatness, the file is next applied. Files vary in length, contour, and section, besides quality of cut, so that several things have to be taken into consideration in their selection. If it is difficult to produce a flat surface on wood by the use of planes, much more so is it the case with files on metal. Steadiness and constant watchfulness are needed to prevent any one part of a surface being reduced too low. The ever-varying position of the file on the work increases the difficulty of maintaining the tool in a horizontal plane, particularly if the surface operated on be small. This is caused by the overhanging ends of the file continually changing position, and, in consequence, altering the amount of lever power, which, if the pressure is not correspondingly varied, must needs give rise to the formation of a curvilinear surface. When filing soft metals, the teeth often get filled up, and are thus prevented from working, in which case a wire brush should be freely used across the file, in the direction of the teeth. A true surface plate, slightly smeared with red lead and oil, will have to be used as a test when the surface approaches towards completion. The parts coming in contact with the surface plate, and therefore requiring to be filed, will be indicated by the red lead markings. After most of the surface shows contact with the test plate, but not so regularly and equally distributed as might be desired, the file should be laid aside, and a flat steel scraper substituted. Cutting, as this tool does, with only one point at a time, there is no difficulty in removing the exact particles desired, and by patient and careful work producing all but a perfect surface.