105 MECHANICS. 



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

 hy observing the degree of steepness required for slid- 

 ing 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 wish to 

 determine the precise amount of force for a wagon 

 weigliing, with its load, one ton, on a plank road. Se- 

 lect some slight descent, where the wagon will barely 

 run with its own weight. Ascertain by a level just 

 what the degree of descent is ; then divide the weight 

 of the wagon by the degree 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 descent of only one foot 

 in fifty feet distance, will be sufficient 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 offered 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 exper- 

 iment. If by such trials as these the farmer ascer- 

 tains the fact that a few inches of mud are sufficient 

 to retard his wagon so much that it will not run of its 

 own weight down a slope of one foot in four (and few 

 common roads are ever steeper), then he may know 

 that a force equal to one fourth the whole weight of 

 his wagon and load will be required to draw it on a 

 level over a similar road — that is, the enormous force 

 of five hundred pounds will be needed for one ton, of 



