216 LECTURE xvnr. 



a direction somewhat ascending, partly on account of the shape of the horse's 

 shoulder, and partly because the principal force that he exerts is in the direci- 

 tion of a line passing through tlie point of contact of his hind feet with the 

 groiind. But a reason equally strong, for having the draught in this direc- 

 tion, is, that a part of the force nicay always be advantageously employed in 

 lessening the pressure on the ground; and to answer this purpose the most 

 effectually, the inclination of the traces or shafts ought to be the same with 

 that of a road on which the carriage would begin or continue to descend by 

 its own weight only. In Order to apply the force in thi.'? manner to both 

 pairs of wheels, where there are four, the line of draught ought to be 

 directed to a point half way between them, or rather to a point immediately 

 under the centre of gravity of the carriage; and such a line would always 

 pass above the axis of the fore wheels. If the line of draught pass immedi- 

 ately through this axis, the pressure on the hind wheels will remain un- 

 altered; and if the traces or shafts be fixed still lower, the pressure 

 on the hind wheels will even be somewhat increased by the draught. 

 It is evident, therefore, that this advantage cannot be obtained if the fore 

 wheels are very high; we may also understand that in some cases the com- 

 mon opinion of the eligibility of placing a load over the fore wheels, rather 

 than the hind wheels, may have some foundation in truth. When several 

 horses are employed, the draught of all but the last must be nearly horizontal; 

 in this case the flexure of the chain brings it into a position somewhat more 

 favourable for the action of the horses; but the same cause makes, the direc- 

 tion of its attachment to the waggon unfavourable; further than this there is 

 no absolute loss of force, but it appears to be advisable to cause the shaft 

 horse to draw in a direction as much elevated as possible; and on the whole 

 it is probable that horses drawing singly have a material advantage, when 

 they do not require additional attendance from the drivers. 



The practice of making broad wheels conical has obviously the disadvan- 

 tageous effect of producing a friction at each edge of the wheel, when the 

 carriage is moving in a straight line ; for such a wheel, if it moved alone, 

 would always describe a circle round the vertex of the cone to which it be- 

 longs. When the wheels are narrow,a slight inclination of the spokes appears 

 to be of use in keeping them more steady on the axles than if they were 

 exactly vertical; and when, by an in clination of the body of the carriage, a 



