Page:Popular Science Monthly Volume 10.djvu/192

 180 the trying-plane following the jack is to remove all the elevations of wood above the valleys the jack left; and, secondly, to compensate by its great length for any want of lineal truth consequent upon the depth of bite of the jack. Again, the mouth of the trying-plane is much narrower than that of the jack; hence the shavings removed are finer; therefore the slope of the iron, or its inclination to the wood, may be less than is the iron of the "jack"—hence the line of cut is 1 more nearly accordant with that of the fibre, and by so much the surface is left more smooth from the trying-plane than from the jack, as there is more cutting and less tearing action than in the jack. The reasoning hitherto pursued in reference to the purpose of this sequence of a jack and trying plane might and does legitimately produce the conclusion that, after the trying-plane has done its duty, the work is as perfectly finished as it can be. Custom, and perhaps other considerations, have established that after the long trying-plane must follow the short and almost single-handed smoothing-plane. So far as the form of the iron of the smoothing-plane is concerned, there is no difference between it and the one used in the trying-plane; each (as across the plane) is straight, the corners being very slightly curved, but only so much as to insure that they do not project below the line of the cutting edge.



The facet edge and inclination of the cutter to the work, and the position of the back-iron, are now under consideration.

It would seem that, while the trying-plane leveled down all the elevations left by the jack, and brought the surface of the wood as a counterpart of that of the plane, there might be, in the fibre or grain of the wood, twists, curls, and other irregularities, which, while leveled, were yet left rough in consequence of the direction in which the cutting edge came upon them. Indeed, this cutting edge in a long plane, which must advance in the direction of its length, must at times come across a large number of surfaces where the fibre is in opposite directions. The consequence is, that there will be various degrees of smoothness; for good work these must be brought to uniformity. This is effected by passing a short-soled plane over the respective parts of the surface in such directions as observation may suggest. Hence the smoothing-plane is of use chiefly to compensate for such