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46 MECHANICS. [Chap. 2.

Thus, if a parallelopiped, say of four inches in widthand one in thickness, as F. plate II. fig. 13, be madesmooth, and laid upon a smooth plane A. B. C. D. andthe weight P. hung over a pulley, it will require theweight P. to draw the body F along, to be equal, whetherit be laid on its side or on its edge.

The experiments of Vince led him to conclude thatthe law, as thus laid down, was * not correct; butthose more recently performed justify the conclusion,that it is so, the deviations being so trifling, as not toaffect the general result.

4. The friction is greater after the bodies have beenallowed to remain for some time at rest, in contact witheach other, than when they are first so placed; as forexample, a wheel turning upon gudgeons will require agreater weight to start it after remaining for some hoursat rest, than it would at first.

The cause of this appears to be, that the minute aspe-rities which exist even upon the smoothest bodies, gradu-ally sink into the opposite spaces, and thus hold uponeach other.

It is for the same reason, that a greater force is re-quired to set a body in motion, than to keep it in motion.If about J the amount of a weight be required to movethat weight along in the first instance, £ will suffice tokeep it in motion.

5. The friction of axles does not at all depend upontheir velocity; thus, a rail-road car travelling at the rateof twenty miles an hour, will not have been retarded byfriction, more than another which travels only ten milesin that time.

It appears, therefore, from the three last laws, thatthe amount of friction is as the pressure directly, with-out regard to surface, time, or velocity.

6. Friction is greatly diminished by unguents, andthis diminution is as the nature of the unguents, withoutreference to the substances moving over them. The kindof unguent which ought to be employed depends prin-cipally upon the load; it ought to suffice just to preventthe bodies from coming into contact with each other.The lighter the weight, therefore, the finer and morefluid should be the unguent, and vice versa.