The use of the file is undoubtedly more difficult than that of the generality of mechanical tools, and the difficulty arises from the circumstance of the file possessing, but in a very inferior degree, the guide principle, the influence of which principle, in all tools, from the most simple cutting tool used by hand, to the most complex cutting machine or engine, formed the subject-matter of the introductory chapter of the present volume. The comparative facility of the manipulation of turning-tools, was shown to depend on the perfection in which the guide principle exists in the turning lathe. It was further stated at page 468 -

"The guide principle is to be traced in most of our tools; in the joiner's plane it exists in the form of the stock or sole of the plane, which commonly possesses the same superficies that it is desired to produce. For instance, the carpenter's plane used for flat surfaces is itself flat, both in length and width, and therefore furnishes a double guide. The flat file it somewhat under the same circumstances, but as it cuts at every part of its surface, from thousands of points being grouped together, it is more treacherous than the plane, as regards the surface from which it derives its guidance, and from this and other reasons it is far more difficult to manage than the carpenter's plane."

• Since writing the above, the author learns that Captain Ericcson tried some experiments on cutting file teeth as with a graver, but that he was led to consider the mode less practical than that of cutting teeth by percussion. The subject appears, however, to deserve more extended trial.

These points arc recalled not to impress the amateur with the idea that the successful use of the file will be to him unattainable, but rather to call forth such a measure of perseverance, as may enable many to arrive at a practice which is confessedly difficult. It is proposed in the present section to notice certain preliminary and general topics, before attempting, in the next three sections, to convey the instructions for manipulating the file.

Commencing with the position of the work, it is in all cases desirable that the surface to be filed should be placed horizontally, and the general rule for the height of the work above the ground is, that the surface to be filed should be nearly level with the elbow joint of the workman, and which may be considered to range with different individuals from forty to forty-five inches from the ground. Some latitude is, however, required in respect to the magnitude of the works, as when they are massive, and much is to be filed off from them, it is desirable that the work should be a trifle lower than the elbow; when the work is minute and delicate, it should be somewhat higher, so that the eye may be the better able to add its scrutiny to that of the sense of feeling of the hand, upon which principally the successful practice depends. The small change of height is also in agreement with the three different positions of the individual in the act of filing; for instance.

Firstly. In filing heavy works, or those which require the entire muscular effort, the file varies from about 12 to 24 inches long, and the length of the stroke is from about 10 to 20 inches, or nearly the full length of the file. The operator stands a little distant from his work, with the feet separated about 80 inches, which somewhat lowers his stature; he grasps and thrusts the handle of the file with his right hand, and bears forcibly near the end of the file with that part of his left hand which is contiguous to his wrist, so as to make the file penetrate the work, or hang to it. The general movement of the person is then an alternation of the entire frame upon the knee and ankle joints, the arms being comparatively fixed to the body, the momentum of which is applied to the file.

Secondly. In filing works of medium size, the file varies from about 6 to 12 inches, and the length of stroke is from about 4 to 9; the operator then stands nearer to the work and quite erect, with his feet closer together. The right hand grasps the file handle as before, but the extremity of the file is now held between the thumb and the first two fingers of the left hand, and the general movement is that of the arms, the body being comparatively at rest.

Thirdly. In filing the smallest works the file is less than 6 inches long, and the stroke does not exceed 3 or 4, and sometimes is not one-tenth as much. When the work is fixed, the file is still usually held in both hands as last described, but frequently, in fact more generally, the file is managed with the right hand alone, the forefinger being stretched out as in holding a carving knife, and the work is held upon the support or filing block with the left hand, as will be explained. The act of filing is then accomplished by the movement of the elbow, or even of the fingers alone, but so little is the body moved, that the workman is usually seated as at an ordinary table.

It is apparent, and also true, that the most direct way of producing a flat surface with the file, would be to select a file the face of which was absolutely flat, and that should be moved in lines absolutely straight; but there are certain interferences that prevent these conditions being carried out. First, although it is desirable to employ files that are as nearly straight as possible, and that are also fixed straightly in their handles, yet very few files possess this exactitude of form, and although in the attempt to attain this perfection, some files are planed in the engineer's planing machine before being cut with teeth, still the cutting and the hardening so far invalidate this practice, that few even of these planed files can retain their perfect straightness, and either both sides become in a small degree irregularly tortuous, or the sides become respectively concave and convex. Therefore, as for the sake of argument, it may be almost taken for granted that no files truly possess the intended form, it is better purposely to adopt that kind of irregularity, which the least interferes with the general use of the instrument.