OP WHEEL-CARRIAGES. 101 



allowed them to draw with more horses, and carry LECT. 



iv 

 greater loads, than usual. Their principal objection ^^v^, 



was, that as a broad wheel must touch the ground in a 

 great many more points than a narrow wheel, the fric- 

 tion must of course be just so much the greater ; and 

 consequently, there must be so many more horses than 

 usual, to draw the wagon. I believe that the majority 

 of people were of the same opinion, not considering, 

 that if the whole weight of the wagon and load in it 

 bears upon a great many points, each sustains a pro- 

 portionably less degree of weight and friction, than 

 when it bears only upon a few points ; so that what is 

 wanting in one, is made up in the other ; and therefore 

 will be just equal under equal degrees of weight, as 

 may be shewn by the following plain and easy ex- 

 periment. 



Let one end of a piece of packthread be fastened to 

 a brick, and the other end te a common scale for hold- 

 ing weights : then, having laid the brick edgewise on a 

 table, and let the scale hang under the edge of the table, 

 put as much weight into the scale as will just draw the 

 brick along the table. Then, taking back the brick to 

 its former place, lay it flat on the table, and leave it to 

 be acted upon by the same weight in the scale as before, 

 which will draw it along with the same ease as when it 

 lay upon its edge. In the former case, the brick may 

 be considered as a narrow wheel on the ground ; and in 

 the latter as a broad wheel. And since the brick is 

 drawn along with equal ease, whether its broad side or 

 narrow edge touches the table, it shews that a broad 

 wheel might be drawn along the ground with the same 

 ease as a narrow one (supposing them equally heavy) 

 even though they should drag, and not roll, as they go 

 along. 



As narrow wheels are always sinking into the ground, 

 especially when the heaviest part of the load lies upon 

 them, they must be considered as going constantly up 



