two and four-wheeled Carriages* $&f 



«arriages; and confequently on fuch roads the former deferve 

 the preference, and the more fo the narrower and deeper the 

 ruts. 



Seclion 42. 



When the dimenflons of the wheels and axles are in the 

 common ratio, the difference on roads of the fecond clafs be- 

 tween R and R' will be confiderably greater than for the carri- 

 ages examined fe&ion 40, and to the prejudice of the four- 

 wheeled carriages ; and the more rugged the road the fleeper it 

 mud be, and the higher and heavier muft be the load to make 

 R < R'. On the whole, two-wheeled carriages on roads of 

 the fecond clafs are always preferable in regard to refiftance. 



Seftion 43. 



But on roads of the third clafs, the advantage inclines 

 ftrongly to the fide of the four-wheeled carriages ; and it 

 is decidedly fo when the furface of the road confifts of 

 foft fand, moift tender earth, tough dirt, &c. fo that the 

 hind-wheels run in the ruts formed by the fore-wheels. For 

 here we have unconditionally R < R'. But even in the cafe 

 of dry foft fand or earth, or fluid dirt, where the ruts made 

 by the fore- wheels immediately clofe up again, we (hall have 

 alfo R < R' when the fum of the femidiameter of the fore- 

 and hind-wheels is not much fmaller than the diameter 

 of the wheels of the two-wheeled carriages without regard 

 to P, a and c. In the laft place, if the road be deep and the 

 load heavy and voluminous, the two-wheeled carriages, to 

 the prejudice of the neceffary ftrength, muft have very large 

 wheels to make R < R'. On the whole, therefore, on fuck 

 poads four-wheeled carriages are better. 



Se&ion 44. 



On the roads with finuofities of fection 34 we have R < R' 

 when the angle £ = o, or vanilhes; that is, when the diftance 

 between the excavations is greater than that between the 

 fore and hind-wheels, or when they are placed fo near each 

 other that the hind-wheel finks while the fore one afeends, as 

 hen, figure 7, at F 77 and F. In both cafes, particularly the 

 laft, four-wheeled carriages are to be preferred to two- wheeled^ 

 not only becaufe R < R y , but becaufe the moving power re- 

 mains greater in the former cafe than in the latter, where 

 the continued alteration in the direction of the pole, and the 

 vibrating of the centre of gravity which thence arifes, tend 

 greatly to fatigue the cattle, 



Se&ion 



