166 LECTURE XVIII. 



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 immediately through this axis, 

 the pressure on the hind wheels will remain unaltered ; 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 common 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 direction 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 

 belongs. 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 inclination of the body of the 

 carriage, a greater proportion of the load is thrown on the lower wheel, its 

 spokes, being then in a vertical position, are able to exert all their strength 

 with advantage. The axles being a little conical, in order that they may 

 not become loose, or may easily be tightened as they wear, it is necessary 

 that they should be bent down so that their lower surfaces may be hori- 

 zontal, otherwise the wheels would press too much on the linch pin. For 

 this reason, the distance between the wheels should be a little greater above 

 than below, and their surfaces of course slightly conical. (Plate XVIII. 

 Fig. 228.) 



It has been proposed to fix the wheels to their respective axles, to con- 

 tinue the axles as far as the middle of the carriage only, and to cause 

 them to turn on friction wheels or rollers ; a plan which may succeed if 

 the apparatus is not too complicated for use ; but in fact the immediate 

 friction on the axles is not great enough to render this refinement neces- 

 sary. If both opposite wheels were fixed to a single axis, one of them 

 would be dragged backwards and the other forwards, whenever the motion 

 deviated from a straight line ; and a similar effect actually takes place in 

 those carriages which are supported on a single roller. 



The effect of the suspension of a carriage on springs is to equalise its 

 motion, by causing every change to be more gradually communicated to it 

 by means of the flexibility of the springs, and by consuming a certaili 

 portion of every sudden impulse in generating a degree of rotatory motion. 

 This rotatory motion depends on the oblique position of the straps sus- 



