178 THE POPULAR SCIENCE MONTHLY. 



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 continu- 

 ity of the fibre, and so prevent an accumulative resistance from this 

 cause. Hence, instead of allowing the cut-off fibres to slide up the in- 

 serted 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 utili- 

 ty, 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 special- 

 ties 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 distinc- 

 tions 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. 



Fig. 6. Fig. 7. 



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 curva- 

 ture 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 



