THE ANGLE OF TRACTION, ETC. 233 



it is evident that the longer the trace the more nearly 

 will the lines x a, x b, x c correspond with the line of 

 traction pa, be, 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 b 

 or c. 



The same reasoning applies to a carriage with four 

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

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

 in which the effort of each horse taken singly is exerted, 

 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 obstacles 

 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 

 turning, compactness, and shortening of -the whole 

 vehicle, and the necessity for making frequent stoppages, 

 or, on the other hand, the wear and tear of the horses, 

 are to be decisive. 



