WHEELS. 377 



equal, though in the larger wheel it is more distributed. 

 The bearing of a large wheel preponderates over 

 the bearinor of a small one, and the arc of the circle 

 which meets the surface of the road being greater, it 

 would not sink so deep ; the large wheel would also 

 have the advantage over the small one in rising over 

 eminences. A wheel of eight feet diameter surmounts 

 an obstacle twice as easily as one of four feet ; if it 

 requires a certain power to draw a carriage of a certain 

 weight over a certain obstacle with wheels of any 

 given diameter, it will require wheels of four times 

 that diameter to draw the same carriage over the 

 same obstacle with half that power. And yet, it 

 has been inferred that, supposing the friction of two 

 carriages of equal weight, but of different-sized wheels, 

 to be equal, the low-wheeled one would be drawn 

 up hill, on a smooth plane, much more easily than 

 the high-wheeled one, though on a smooth, horizontal 

 surface the latter would be drawn rnore easily than 

 the former. On the contrary, on going down hill, 

 the high- wheeled carriage will be urged forward 

 by its relative gravity more than the low-wheeled 

 one, consequently low-wheeled carriages are preferable 

 to high-wheeled ones in a hilly country, both for going 

 up hill and down hill. This has been proved with 

 bicycles : where hills have to be ascended and 

 descended, a small wheel has an advantage over the 

 big one. Were large wheels made experimentally 

 and attached to a carriage for the purpose of being 

 driven, were the axles to be raised above the breasts, 

 of the horses, they would have to draw downwards, and 

 would, in consequence, be incapable of exerting their 

 utmost strength ; with small wheels the axle would, 

 being low, increase the draught, in consequence of 

 the horses having to draw upwards. And yet, so far 



