236 DRAUGHT AND HAKKESS. 



angles to the line of traction p a b c, and supposing 

 the effort of traction to be exerted successively at a 5 c, 

 it is evident that the longer the trace the more nearly 

 will the lines x a, x h,xc correspond with the line o 

 traction 2^ a, h c, y, and therefore the more usefully 

 will the power be employed. Nothing wears or fatigues 

 a horse more than sudden lateral shocks to the collar, 

 the whole weight of which falls on the fore legs ; and it 

 is further evident that any obstacle, such as a stone or 

 a rut, impeding one of the wheels, x, will give a greater 

 shock to the horse's collar when at a than when at h 

 or c. 



The same reasoning applies to a carriage with four 

 wheels, and with two or four horses, as is evident from 

 fig. 20, II, which shows that although the lines n o, 

 in which the effort of each horse taken singly is ex- 

 erted, always remain parallel to the line of traction, 

 p y, of the carriage, whatever the length of the trace 

 may be, still the longer trace corresponds more nearly 

 with the line of effort n o than does the shorter 

 one. 



The result of the whole argument is therefore this : 

 on good well-kept roads that present no lateral ob- 

 stacles to the wheels, such as stones or ruts, we inflict 

 much less injury on our horses by short harnessing 

 than would otherwise occur; and be it remembered 

 that it is only on such roads, and in flat countries, 

 that two-wheeled draught is at all admissible and 

 generally employed ; but we always take more out of 

 the horse than is absolutely required for the traction by 

 so doing, and it therefore is a matter for calculation in 

 each case whether considerations of handiness in sharp 

 tui'ning, compactness, and shortening of the whole 



