672 Oil the Advantage of Employing 



the force was 140 pounds with the new wheels, and 

 150 with the old ones. 



A very remarkable circumstance in the results of these 

 experiments, and one which seems to me sufficiently im- 

 portant to deserve to be generally recognized, is the 

 great effect which the nature of the road has upon the 

 relation which the required increase of force bears to 

 any increase of speed. 



We have seen that, when the coach was going at a slow 

 walk over a paved road, the force with the new wheels 

 was only about 40 pounds ; but that, at a slow trot, it be- 

 came equal to 80 pounds, and at a rapid trot it equalled 

 120 pounds. On an unpaved road, however, as well as 

 in sand, the force was always the same, or very nearly 

 so, whatever the speed of the horses might be. This 

 difference no doubt arises from the severe shocks which 

 the carriage receives when it is drawn rapidly over a 

 pavement ; for it is evident that, for each blow which 

 the carriage receives from the stones of the pavement, 

 there is a certain amount of force employed, and this 

 must always be supplied by the horses. From this fact 

 we may draw the important conclusion that, the easier 

 a carriage is to ride in, the less is the force necessary to 

 draw it, its weight and load remaining the same ; and, as 

 no one can doubt that wheels with broad felloes must 

 roll over a pavement more easily than narrow wheels, 

 this fact alone is enough to show that they are prefer- 

 able to the old kind of wheels for all sorts of carriages. 



A knowledge of the remarkable fact that the amount 

 of force required to draw a carriage over an unpaved 

 road is not sensibly increased by increasing the speed 

 might be put into practice with advantage on many 

 occasions in husbanding the strength of the horses. It 



