148 DRAUGHT CH. X 



the weight, and its ability to do so depends ob- 

 viously upon the relative lengths of the two arms 

 CB and AB. CB is the radius of the wheel, and 

 AB will depend upon the size of the obstacle as 

 indicated at B. For the same size of wheel, there- 

 fore, the smaller the obstacles, or, in other words, 

 the smoother the road, the less will be the resist- 

 ance to rolling, and for the same roughness of road 

 the larger the wheel, the less will be the resistance. 



The length of the arm AB, in the case of the small 

 obstacles, which make up the roughness, will be 

 practically the same as the arc AB, and will be 

 measured by the angle BCA. For any given sur- 

 face of road, this arc may be represented by a con- 

 stant, which, multiplied by the radii of various wheels, 

 will measure the rolling resistance. Theoretically, 

 therefore, this resistance should be inversely propor- 

 tional to the diameter of the wheel. In 1838- 1840, a 

 series of elaborate experiments made by Morin * for 

 the French Government, on the Traction of Vehicles, 

 confirmed this as a practical fact ; it may be there- 

 fore laid down as a law, that on any given road, 

 the rolling friction will diminish directly in propor- 

 tion as the size of the wheel is increased. f If 



* Notwithstanding the comparatively early date of these experi- 

 ments they are still the most satisfactory that we have, and the most 

 complete, having been made on a very large scale, with heavy vehi- 

 cles, on all kinds of roads ; later experiments have only confirmed 

 them. 



f Dupuit and other writers maintained that resistance diminished 



