Page:Popular Science Monthly Volume 10.djvu/190

178 possible. This led to a contrivance now almost universal, that of breaking the fibre so soon as it is separated from the piece of timber. The designer seems to have considered that, as soon as a short length of shaving had been removed, it would be well to destroy the continuity of the fibre, and so prevent an accumulative resistance from this cause. Hence, instead of allowing the cut-off fibres to slide up the inserted chisel, he bent them forward, in fact, cracked them, and so broke the cumulative indrawing force of them. This he accomplished by the use of what is now called the back-iron, and henceforth the boxed-in chisel loses its identity, and must be regarded as part of an independent tool.

The tool thus built up is called a plane, and from its general utility, and capability of adaptation to various forms and conditions, it is well deserving of the high opinions entertained of its powers. Three forms of this tool are in general use in English workshops, called the "jack," the "trying," and the "smoothing" planes. These are on the bench of all workers in smooth, straight-surface wood. Although externally alike except in size, they are yet used for different purposes, and each has a specialty met with in its construction. These specialties may now be considered.

After the wood has passed from the sawyer into the hands of the carpenter, the surface undergoes those operations which render it true and smooth. These three planes do this work. The "jack," usually about fifteen inches long, and the "trying" plane, ranging from eighteen inches to twenty-four inches long, but, in exceptional cases, far exceeding these dimensions, are to external appearances alike; indeed, some regard the different handles as the only distinctions between them, and that these handles show which must be used for rough work and which for smooth (see Fig. 8 as an example of the handle of a "jack-plane," and Fig. 9 as an example of a "trying-plane handle"). This is an error. There are other differences, but the main and leading one is the different form given to the edge of the cutting-iron.



If the iron of the "jack" plane be looked at from the front end of the plane, the form of the edge will be curved, as in Fig. 6; but the iron of the "trying" plane is straight, as in Fig. V. Upon the curvature of the edge depends the efficient action of the "jack."

Sufficient has been said of the tendency of the fibre to draw the tool downward; but it must not be forgotten that the same adhesion