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itself forward, or by which the progressivemotion of the engine is effected, is, therefore,the adhesion of the surface of the wheels uponthe rail; and the reader will perceive that thispower is acquired without in the least affectingor adding to the resistance. The resistance, orfriction of the wheels upon the rail, would bethe same upon the wheels of any other car-riage, of the same weight, as upon the wheelsof the engine when propelling itself forward;and this is not mere supposition, for, in com-paring the space passed over by the engine,with a given number of revolutions of thewheels, no sensible distance was lost, whichproved that no sliding took place.
By knowing the amount of adhesion uponsurfaces, in contact with each other, we mightdeduce the power of adhesion presented bythe rail to the wheels of the engine, and thiswould be the power required to cause theengine to slide along upon the rail, supposingthe wheels were prevented from turning round.This, however, is by no means constant; atone time the rails are quite dry, at anotherquite wet, at other times partly both, andsometimes slightly covered with a film of mud;in all of which cases the amount of adhesionvaries; to deduce to any data of practical uti-lity, we must, therefore, ascertain the amount