THE ANGLE OF TKACTION, ETC. 239 



sixteen hands high ; the point of attachment of the 

 trace to the hames could with this sized animal be 

 scarcely brought nearer to the ground than 44 inches, 

 and with a horizontal trace this would involve a 

 fore-wheel of 7 feet 4 inches in diameter, something 

 quite out of the question for carriages, the highest 

 wheel used for field-guns being 5 feet in diameter. It 

 is, therefore, quite impossible to carry out this principle 

 to its fullest extent even with the horizontal trace, which 

 we will admit to be the most favourable for traction 

 on a perfect level and smooth surface, and putting this 

 slanting direction of the horse's shoulder for the present 

 out of the question. 



But we have already pointed out that when the 

 carriage leaves the level and gets on to an inclined 

 plane, the horizontal trace becomes parallel to the road, 

 and is no longer at right- angles with the perpendicular 

 passing through the nave of the wheel, and we shall 

 now proceed to show what happens when the wheel 

 meets an obstacle such as a stone, or gets into a rut, the 

 road being otherwise level. Fig. 22 shows two circles, a 

 smaller and a greater one, representing two wheels of 

 unequal size touching the ground at the point A, and 

 each just in contact with two obstacles N N ' of precisely 

 equal height. In order to enable each wheel to surmount 

 the obstacle before it, a certain amount of power must 

 be applied to the- axles O and O', and this will act 

 respectively on the bent levers M A ', and O ' M ' A ', 

 and in both cases will be most advantageously applied 

 in the direction Q and Q ', at right-angles to O M and 

 O ' M '. Therefore the horizontal pull on the trace is 

 not in this case a very common one the most ad- 

 vantageous, but the contrary. It is, however, evident 



