THE ANGLE OF TEACTION, ETC. 241 



On the other hand, we have seen that with a sixteen- 

 hand horse, a horizontal trace is equivalent to a wheel 

 of 7 feet 4 inches in diameter, a dimension quite un- 

 suited to ordinary purposes ; and although such a 

 wheel would surmount an obstacle of a given height 

 more easily than a 5-foot wheel, there would be a loss 

 of power, or rather an unnecessary expenditure of it 

 demanded from the horse, because the effort could not 

 be made in the direction Q, and therefore the sixteen- 

 hand horse would experience less fatigue when working 

 5-foot wheels than with the otherwise more powerful 

 7-foot 4-inch ones, and least of all, of course, when his 

 traces were attached to the middle of the hames, not only 

 because this would.bring the line of traction nearer to O Q, 

 but also and mainly because it would suit his own con- 

 formation and the mechanism of his movements better. 



In order to wind this matter up, let us remind our 

 readers of one more very simple and familiar fact 

 namely, that plough-horses do all their work under a 

 very considerable angle of traction, and in this case 

 there was no wheel to mislead people with Will-o'-the- 

 Wisp theories, or at least not until lately. 



There is one other point in connection with wheels 

 to which we desire to call attention, because this too is 

 frequently misunderstood. It is said that, with a four- 

 wheeled vehicle, the load should be put as far forward 

 as possible, and that a man placed at the hinder end of 

 a carriage gives the horse as much work to do as two 

 men placed in front. 



Now we have just seen that the lever-power of any 

 two wheels in overcoming obstacles and, except on a 

 railway, there are always obstacles to overcome is 

 proportionate to their height or diameter ; but in most 



