152 MOTIVE POWER AND DISPOSITION

cylinders is employed to turn the wheels of theengine round, by a pin fixed in one of thespokes of the wheel, and acting as a commoncrank; these wheels, resting on the rail, pressupon them with a force equal to the weight ofthe engine, divided into the number of wheels;thus, suppose there are four wheels, then thepressure of the periphery of each wheel, uponthe surface of the rail, is equal to one-fourthof the weight of the engine. Now it is wellknown, that when two surfaces are in contact,and subjected to a certain pressure, it requires acertain force to cause them to slide upon eachother; that property, in bodies, to resist'slid-ing, is called adhesion of the surfaces. Sup-pose, then, the power of the cylinders be em-ployed to turn the wheels of the engine round,this property of bodies, in contact, will preventthe wheels of the engine from sliding upon therail, with a force equal to their weight orpressure, and they will necessarily roll along

the surface. If, however, the progressive

motion be opposed to a retarding force, greaterthan the force or power of adhesion, then thesurfaces will slide upon each other; and therotatory motion of the wheels will be continuedwithout effecting a progressive motion of theengine.

The power by which the engine propels