CH. Ill WHEEL 39 



at D. The longer the arm B C, and the shorter the 

 arm C D, the more easily will the draught raise the 

 weight of the wheel, and for the same sized obstacle 

 the larger wheel will evidently have the advantage. 



In the figure, the lines of draught make the same 

 angle with the radius-arm of the lever ; if the lines 

 of draught are parallel, the advantage of the larger 

 wheel is still greater. 



There is a limit in practice to the size of wheels, 

 because, if very large, their weight may more than 

 balance their other advantages ; but this will be true 

 only of extravagant designs ; within the usual limits, 

 the larger the wheel the better. The very small 

 wheels sometimes used on pony phaetons are cruel 

 to the horses. 



On a road which is level crosswise, the wheel 

 which will run the easiest must be straight, vertical, 

 with a tire at right angles to the face of the wheel, 

 and the axle must be cylindrical, — that is, not ta- 

 pered, — and horizontal. In other words, the wheel 

 must be a short section of a true cylinder, revolving 

 on a horizontal cylindrical axis. All rolling frames or 

 carriages in ordinary machinery are thus mounted. 



In wagon and carriage building, departures from 

 this system have been made for various reasons. 



In the first place, it is much more easy to make a 

 wheel fit on a taper axle-arm than on one with par- 

 allel sides, since the application of washers behind 

 the wheel will determine how far it may be pushed 

 on the arm and how tight it may be ; while with a 



