66 WHEELS AND THE UNDER-CARRIAGE. 



full-sized scale drawing is rendered upon a blackboard from 

 which the working drawing is made. 



WHEELS. 



The wheels are usually given a " dish," i. e., all of the 

 spokes are bent slightly outward, so that when the tire is put 

 on it does not line with the centre of the nave. The extent 

 of this curve has been determined by what experience has 

 shown to be productive of the best results. More strength 

 and elasticity are thus given to the wheels. When the wheel 

 is given this dish, the arm or end of the axle is correspond- 

 ingly bent downward from the shoulder. Unless this is 

 done, the dishing of the wheel acts, in a degree, as an im- 

 pediment in traction. 



THE UNDER-CARRIAGE. 



The under-carriage is formed of various sized timbers 

 which connect the four wheels and offer a support for the 

 body. These timbers have no broad surfaces, as they are as 

 deep or deeper than they are wide, and are joined by dowel- 

 ling, iron braces, bolts and so forth. The under-carriage is 



a-g 



B B 



FIG. 35. 

 A ROLLER BOLT'S. B FUCHELLS. 



of three kinds, the long perch, demanding much room in 

 turning, the short perch and the crane neck ; the latter 

 two were devised to facilitate the performance of this require- 



