MECHANICS. 99 



percussion to overcome friction, compared with mere 

 pressure or weight. 



163. When motion begins, the intensity of fric- 

 tion diminishes ; it does not, however, change after- 

 wards as the velocity changes, but continues, as al- 

 ready said, to retard with a uniform force. 



EULER makes the friction to be reduced to one-half, 

 when the body is actually put in motion. Sur le 

 Frottement des Corps Solides, Mem. Acad. de Berlin, 

 1748, p. 122, &c. 13. The reduction appears in 

 some cases to be much greater than this. COU- 

 LOMB found the friction of wood sliding on wood to 

 become less \\hen the body began to move, than it 

 had been the instant before, in the ratio nearly of 

 2 to 9. Its intensity afterwards did not change. 

 PHONY, Arch. Hydraulique, 1173, N 2. 



164. Friction may be measured by finding the 

 force necessary to bring a body resting on a hori- 

 zontal plane into such a state that it is ready to 

 move on the application of the least force ( 153.) : 

 it may also be measured by placing the body on a 

 plane of variable inclination, and increasing that 

 inclination till the body begin to slide. As the ra- 

 dius to the tangent of that inclination, so the 

 weight of the body to its friction on the horizontal 

 plane. 



If the weight be W, and the inclination of the plane, 

 when the body begins to slide, t, the friction 

 W x tan ?*. 



G 2 165. Time 



