102 FERGUSON'S LECTURES. 



LECT. hill, even on level ground. And their sides must sus- 

 iv. 

 .x>v-xy tain a great deal of friction by rubbing against the ruts 



made by them. But both these inconveniences are 

 avoided by broad wheels ; which, instead of cutting 

 and ploughing up the roads, roll them smooth, and har- 

 den them ; as experience testifies in places where they 

 have been used, especially either on wettish or sandy 

 ground: though, after all, it must be confessed that 

 they will not do in stiff clayey cross roads ; because 

 they would soon gather up as much clay as would be 

 almost equal to the weight of an ordinary load. 



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 ; that is, to stand at right 

 angles 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 

 become 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 

 not be at their full strength. So that the usual way 01 

 making the wheels concave is by much the best. 



The axles of the wheels ought to be perfectly straight, 

 that the rims of the wheels may be parallel to each 

 other ; for then they will move easiest, because they will 

 be at liberty to go on straight forwards. But, in the 

 usual way of practice, the axles are bent downward at 

 their ends ; which brings the sides of the wheels next 

 the ground nearer to one another than their opposite 

 or higher sides are : and this not only makes the wheels 

 to drag sideways as they go along, and give the load a 



