THE STEAM-ENGINE. 531 



in which the public have availed themselves of these means ; and of the im- 

 provements of which they seem to us to be susceptible. 



It is a singular fact, that in the history of this invention considerable time 

 and great ingenuity were vainly expended in attempting to overcome a diffi- 

 culty, which in the end turned out to be purely imaginary. To comprehend 

 distinctly the manner in which a wheel-carriage is propelled by steam, suppose 

 that a pin or handle is attached to the spoke of the wheel at some distance from its 

 centre, and that a force is applied to this pin in such a manner as to make the 

 wheel revolve. If the tire of the wheel and the surface of the road were 

 absolutely smooth and free from friction, so that the face of the tire would slide 

 without resistance upon the road, then the effect of the force thus applied would 

 be merely to cause the wheel to turn round, the carriage being stationary, the 

 surface of the tire slipping or sliding upon the road as the wheel is made to 

 revolve. But if, on the other hand, the pressure of the face of the tire upon 

 the road is such as to produce between them such a degree of adhesion as will 

 render it impossible for the wheel to slide or slip upon the road by the force 

 which is applied to it, the consequence will ,be, that the wheel can only turn 

 round in obedience to the force which moves it by causing the carriage to ad- 

 vance, so that the wheel will roll upon the road, and the carriage will be moved 

 forward, through a distance equal to the circumference of the wheel, each time 

 it performs a complete revolution. 



It is obvious that both of these effects may be partially produced ; the ad- 

 hesion of the wheel to the road may be insufficient to prevent slipping alto- 

 gether, and yet it may be sufficient to prevent the wheel from slipping as fast 

 as it revolves. Under such circumstances the carriage would advance and 

 the wheel would slip. The progressive motion of the carriage during one 

 complete revolution of the wheel would be equal to the difference between the 

 complete circumference of the wheel and the portion through which in one 

 revolution it has slipped. 



When the construction of travelling steam-engines first engaged the attention 

 of engineers, and for a considerable period afterward, a notion was impressed 

 upon their minds that the adhesion between the face of the wheel and the sur- 

 face of the road must necessarily be of very small amount, and that in every 

 practical case the wheels thus driven would either slip altogether, and produce 

 no advance of the carriage, or that a considerable portion of the impelling 

 power would be lost by the partial slipping or sliding of the wheels. It is 

 singular that it should never have occurred to the many ingenious persons who 

 for several years were engaged in such experiments and speculations, to as- 

 certain by experiment the actual amount of adhesion in any particular case 

 between the wheels and the road. Had they done so, we should probably 

 now have found locomotive engines in a more advanced state than that to which 

 they have attained. 



To remedy this imaginary difficulty, Messrs. Trevethick and Vivian pro- 

 posed to make the external rims of the wheels rough and uneven, by surround- 

 ing them with projecting heads of nails or bolts, or by cutting transverse 

 grooves on them. They proposed, in cases where considerable elevations 

 were to be ascended, to cause claws or nails to project from the surface during 

 the ascent, so as to take hold of the road. 



In seven years after the construction of the first locomotive engine by these 

 engineers, another locomotive engine was constructed by Mr. Blinkensop, of 

 Middleton colliery, near Leeds. He obtained a patent, in 1811, for the appli- 

 cation of a rack-rail. The railroad thus, instead of being composed of smooth 

 bars of iron, presented a line of projecting teeth, like those of a cog-wheel, 

 which stretched along the entire distance to be travelled. The wheels on 



