1 52 Statics. 



powder, it acquires, at the same time in consequence of the 

 friction against the bottom of the bore, a rotatory motion, which 

 is but little affected while in the air ; but when the ball comes to 

 reach the ground, as the rotatory motion on the part toward the 

 ground takes place in a direction opposite to the progressive mo- 

 tion, the consequence must be an acceleration in the motion of 

 248. the centre, that is, of the progressive motion. 



250. Finally, if friction is disadvantageous in many cases, it 

 is still not without its utility. Were it not for this, the least in- 

 clination would be continually subjecting us to a fall. No man 

 or other animal could turn while in rapid motion without falling, 

 F Jg-136 wna tever position he might take ; whereas, on account of friction, 

 an animal may incline himself toward the point .F, for example, 

 about which he is moving, in such a manner that his weight, di- 

 rected according to the vertical GK, passing through the centre 

 of gravity G, and the tendency to fly off GC, acquired by turn- 

 ing, and which is directed from F toward C, will conspire to pro- 

 duce a single force according to the line G7, passing through a 

 point / between the legs of the animal ; this force, although ob- 

 lique, is still destroyed by friction, provided the inclination be 

 within the limits required by the laws of friction. 



It is moreover to friction that we are indebted for the power 

 of diminishing friction, when injurious ; since it is only by means 

 of this resistance that we are able to work and polish the surfa- 

 ces of bodies. It is to friction that we owe the facility with which 

 the parts of certain machines are rendered sometimes fixed, and 

 sometimes moveable. It is by friction that scissors and the like 

 instruments, pincers, forceps, files, &c., produce their effect. If the 

 parts of scissors, for example, were not saws, armed with small 

 teeth, which take into the cavities of the bodies to be cut, these 

 bodies would slip from between the two edges. 



Friction is also very often of service in moving bodies in 

 certain directions ; thus if we would raise the body p by means 

 of the lever AB, it is very easily done, by making the body bear 

 Fig 137. on the edge CD. The friction in a case like this, being very 

 considerable, renders CD fixed, and prevents the body from slip- 

 ing. The same cause keeps the extremity A of the lever in its 



