MORE CONCERNING MECHANICAL TOOLS. 179 



of fibre to fibre takes place between the surface-fibres as among those 

 below the surface. If tools excluded from this course of lectures had 

 entered, we should have found that these connecting surface-fibres 

 are separated by the addition of certain supplementary appendages to 

 the tools. The depth to which the plane penetrates has led to the 

 combination in one edge of such supplementary parts. 



For the purpose of separating the surface connecting fibres, the 

 jack-iron is convex. Note now its action. The convex sharp edge 

 is pushed along an horizontal plank, penetrating to a depth determined 

 by the projection of each vertical section below the sole of the plane. 

 The ends of this convex edge are actually within the box of the plane ; 

 consequently (sideways) all the fibres are separated by cutting, and 

 are therefore smooth and not torn. The effect of this upon the entire 

 surface is to change the surface from the original section to a sec- 

 tion irregularly corrugated. The surface after using the "jack" is 

 ploughed, as it were, with a series of valleys and separating hillocks, 

 the valleys being arcs from the convexity of the tool, and the separat- 

 ing hillocks being the intersections of these arcs. All traces of the 

 tearing action of the saw have been removed, and from a roughened 

 but level surface a change has been made to a smooth but in cross- 

 section an undulating one. 



The mechanician's next object is to remove these lines of separa- 

 tion between the valleys. For this, the trying-plane is required. The 



Fig. 8. 



trying-plane is longer than the jack, because the 6ole of the plane 

 which is level is, so far as its size goes, the counterpart of that which 

 the surface of the wood is to be ; further, the trying-plane should be 

 broader than the jack, because its object is to remove the valleys, 

 and not to interfere with the wood below the bottoms of the valleys. 

 If its action passes below the bottoms of the furrows, then occasion 

 arises for cutting the side-connection of the fibres, and, however a 

 workman may sharpen the edge of his trying-plane for this purpose, 

 he in one respect has destroyed one object of the plane, because, so 

 soon as the iron penetrates below the surface, so soon does the effect 

 of the jack-action begin to reappear, and the cutting edge should pass 

 from the shape shown in Fig. 8 to the shape in Fig. 7. The result of 



