84 MECHAJ^ICS. 



a heavy load upon a sled in winter for crossing a bare 

 wooden bridge or a dry barn floor, the friction between 

 cast-iron sleigh-shoes and rough sanded plank being nearly 

 equal to one-third of the whole weight.* Hence a load 

 of one ton (including the sled) would require a draught 

 equal to more tlian six hundred pounds, which is too much 

 for an ordinary single team. On bare unfrozen ground the 

 friction would be still greater. On a plank bridge, with 

 runners wholly of wood, it would be equal to half the 

 load. All these facts may be readily proved by actually 

 placing the sled on slopes of plank and of earth, and by 

 observing the degree of steepness required for sliding 

 down by its own weight. 



In a similar way, we are enabled easily to ascertain the 

 force required to draw a wagon upon any kind of level 

 surface. Suppose, for example, that we w^ish to determine 

 the precise amount of force for a wagon weighing, with 

 its load, one ton, on a plank road. Select some slight de- 

 scent, where the wngon will barely run with its own 

 weight. Ascertain by a level just what the degree of de- 

 scent is ; then divide the weight of the wagon by the de- 

 gree of the slope, and we shall have the force sought for. 

 To make this rule plainer by an example : It will be found 

 that a good, newly-laid plank track, if it possess a de- 

 scent of only one foot in fifty feet distance, will be suflS- 

 cient to give motion to an easy-running wagon ; therefore 

 we know that the strength required to draw it on a level 

 will be only one-fiftieth part of a ton, or forty pounds. 



The resistance oflfered to the motion of a wagon by a 

 Macadam road, by a common dry road, and by one with 

 six inches of mud, may be readily determined in the same 

 way by selecting proper slopes for the experiment. If by 

 such trials as these the farmer ascertains the fact that a 



* On clean hard wood, with polished metallic shoes, the friction 

 would be much less, or a fourth or fifth. 



