94 OUTLINES or NATURAL PHILOSOPHY. 



The subject of friction is treated by MUSCHENBROEK, 

 510,^533. BOSSUT, 248, 283. PHONY, Arch. 

 Hydrauliqu?., Sect. v. BREWSTER, in his Appendix 

 to FERGUSON'S Mechanics, vol. u. p. 334. GREGQ- 

 RY'S Mechanics, vol. u. 24, &c. 



7< When a lieavy body is at rest oil ft hori- 

 zontal plane, it is not in equilibria, or ready to 

 obey the least impulse, in a direction parallel to 

 the plane. A force must be applied to bring it 

 into that state, with respect to any given direc- 

 tion ; and this force is the measure of the fric- 

 tion. 



158. Friction destroys, but never generates mo- 

 tion, and in this is unlike gravity, or any of the 

 forces hitherto considered, which, if they retard 

 motion in one direction, always accelerate it in 

 the opposite. The force of friction, therefore, vio- 

 lates the law of continuity, and cannot be accu- 

 rately expressed by any geometrical line, or any 

 algebraic formula. 



a. The retardation which friction opposes to motion, 

 is nearly uniform, or the same for all velocities. 

 COULOMB, in a series of experiments made with 

 great accuracy, and on a large scale, found that 

 bodies sliding on a plaile, oil which they Were tBO- 

 ved by a constant force of IrdClwn^ were uniformly 

 accelerated, which could not have been, if the im* 

 peding as well as the impelling force had not been 



constant. 



