128 MOTIVE POWER AND DISPOSITION

only one cylinder, and, from what I can learn,did not materially differ, in construction, fromthat previously described, except in the formof the carriage.

The great obstacle to their introduction atthat time, was the supposed want of hold oradhesion of the wheels upon the rails to effectthe loco-motion of the engine. Messrs. Trevi-thick and Vivian, in their patent, recommendedmaking the external periphery of the wheelsrough or uneven, by using projecting heads ofnails, bolts, or cross-grooves; or, in case ofa hard pull, to cause a lever, bolt, or claw toproject through the rim of one or both of thesaid wheels, to take hold of the ground. Butit will appear obvious to any one, that thismode of remedying one defect would be themeans of producing another ; for any projec-tions would not only cause considerable resist-ance to the progressive motion of the engine,but would also tend to injure the rails of theroad.

To obviate these defects, Mr. Beenkinsop,of Middleton colliery, near Leeds, in 1811,obtained a patent for the application of a rack,or toothed rail, stretched along the whole dis-tance to be travelled, into which wheels, turnedby the engine, worked, and thus produced aprogressive motion in the carriage.