WHEEL CARRIAGES. 105 



avoided by using broad wheels ; which, instead of 

 cutting and ploughing up the roads, roll them 

 smooth, and harden them, as experience testifies 

 in places where both have been used. 



If the wheels were always to go upon smooth 

 and level ground, the best way would be to 

 make the spokes perpendicular to the naves and 

 to the axles ; because they would then bear the 

 weight of the load perpendicularly, which is the 

 strongest way for wood. But because the ground 

 is generally uneven, one wheel often falls into a 

 cavity, or rut, when the other does not ; and then 

 it bears much more of the weight than the other 

 does ; in which case, concave, or dishing-wheels, 

 are best ; because when one falls into a rut, and 

 the other keeps upon high ground, the spokes be- 

 come perpendicular in the rut, and therefore have 

 the greatest strength when the obliquity of the 

 load throws most of its weight upon them ; whilst 

 those on the high ground have less weight to bear, 

 and therefore need be at their full strength : so 

 that the usual way of making the wheels concave 

 is best. 



