10 



Laboratory Arts 



across, the wood then being reversed, and the rest planed. 

 Otherwise, the wood will crack down as the plane reaches the 

 end portion of the stroke (Fig. 2). A simple method of 



overcoming this difficulty is by 



crack { ^ the use of a " shooting-board," 



which provides a stop against 

 which the wood rests, and thus 

 receives the support necessary 

 to prevent splitting. 



The rough surface being re- 

 moved with a jack plane, a 

 trying plane (or the jack plane 

 set more finely) is used to make 

 that surface fairly flat. A 

 FlG 2 smoothing plane then takes out 



the plane marks previously made. 

 It must therefore be very finely set. 



The surface so produced may be still further finished by 

 the use of glass paper. 



Saws. Many kinds of saw exist, but for the purposes 

 of apparatus making only three need be mentioned : (a) the 

 hand saw ; (b) the tenon saw ; (c) the bow saw. 



(a) The hand saw is a long saw, usually constructed with 

 teeth that permit cutting either with or across the grain. It 

 is used when dealing with large pieces of timber, but should 

 not be used when sawing thin, broad pieces, being a heavy tool, 

 and liable to split the wood when this is disproportionately 

 light. If it be desired to cut up strips of wood along the 

 grain, a special saw (the rip saw) is used, but this is a still 

 heavier tool, with coarsely set teeth, and requires practice in 

 handling. 



(b) The tenon saw is the most useful tool from our point 

 of view. This saw is made of hard steel, and is kept rigid 

 by a back plate, either of steel or brass. The teeth are small, 

 and cut best across the grain, but are able to cut in any 

 direction. 



(c) The bow saw is used for cutting along curved lines. 

 The saw blade is very narrow, and is kept rigid by the tension 



