ON HAH.-IIOADS.

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required to keep the waggon in motion, or tokeep it in a state of uniform velocity, thatvelocity being first produced by other means.Before, however, proceeding' further, it maybe necessary to define what is meant by fric-tion, and explain the laAvs which regulate itsaction upon bodies moving upon Rail-roads.—As previously stated, when two surfaces are incontact, and subjected to a determinate pres-sure, it requires a certain force to cause themto slide over each other; and this property ofresisting sliding, was called the adhesion ofthe surfaces. Friction, then, is meant as ameasure of this resistance, or the amount offriction is the force required to cause the bodiesto slide over each other.

It is a well-known law of dynamics, thatevery body has a tendency to continue eitherin a state of rest, or of uniform rectilinearmotion, unless disturbed by some extraneousmechanical force. Let, therefore, a carriagebe placed upon a Rail-road, and a certainvelocity be given to it, by any force or pressurewhatever — if we suppose the body and planeperfectly void of friction, and free from theaction of any other force, then the body willcontinue to move on uniformly with the velo-city given to it, until disturbed by some acci-dental force or cause ; for the body lias no

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