First in importance among the mechanisms which demand attention here is the brake. From the beginning of railroads the necessity for brakes was apparent, and in 1833 Robert Stephenson patented a steam driver-brake (the brake on the driving-wheels). This was but four years after the Rainhill trials, which settled the question of the use of locomotives on the Liverpool & Manchester Railroad. This early brake contained the principle of the driver-brake, operated by steam or air, which has in late years come into wide use. The apparatus is so simple that the cut representing it hardly needs explanation. Admission of steam into the cylinder raised the piston, which through a lever and rod raised the toggle-joint between the brake-blocks and forced them against the treads of the wheels. Essentially the same method of applying the retarding force can now be seen on most passenger engines, and often, but not so commonly, on engines for freight service. For various reasons Stephenson's driver-brake did not come into use. [Illustration: English Screw-brake, on the Birmingham and Gloucester Road, about 1840.] Innumerable devices for car-brakes have been invented, but they divide themselves into two groups: those in which the retarding force is applied to the circumference of the wheel, and those in which it is applied to the rail. The class of brakes in which the retarding force is applied to the rail has been little used, although various contrivances have been devised to transfer a portion of the weight of the car from the wheels to runners sliding on the rails. There are many objections to the principle, and it will probably never again be seriously considered by railroad men. The apparatus is necessarily heavy, the power required to apply it is great, and its action is slow. When brought into action it is not as efficient as the brake applied to the tread of the wheels, and the transfer of the load increases the chance of derailment. [Illustration: English Foot-brake on the Truck of a Great Western Coach, about 1840.]