Year by year the locomotive was being perfected in all its details. The cab made its appearance and was first opposed by the engineers, who imagined that they would be badly penned in, in case of accident. The Erie contributed the bell-rope signal from the train; we have already heard of that first whistle on the locomotive of the Sandusky and Mad River Railroad. The Boston & Worcester devised the headlight, so that time might be saved by handling freight at night. More important than these were the experiments by Ross Winans and by S. M. Felton that led to the substitution of coal for wood as a fuel, and the development by Rogers at his Paterson works of the link device, so necessary in stopping, starting, and reversing the locomotive. Gradually the size of the locomotive increased to 28 and 30 tons in the late fifties. Finally James Milholland, engineer of machinery for the Philadelphia & Reading Railroad, built in 1863 a pusher engine for coal trains that weighed something over 50 tons. When folk saw that engine they almost gasped, and wondered what the railroads were coming to. But the wiser men kept silent. They knew that as long as bridges and roadbeds and fine steel rails were increased in strength, the limit of size of the locomotive had not been reached. The greater grip the locomotive has upon the rail, the greater its pulling power, the greater its efficiency. Sheer weight, and weight alone, gives that grip. It certainly takes a weight of seven tons to give a grip of one ton upon a dry rail; in the case of wet rails this ratio becomes ten to one. Then wonder not that the locomotive steadily increased in size, that the Moguls with six driving-wheels, and the Consolidations with eight, came into vogue a few years after the close of the war, and that these kept increasing in weight all the while. Height and width were and still are rigidly limited by the clearance of the line. The locomotive must stand no more than fourteen or sixteen feet high and from nine to eleven feet wide; in length the problem only meets the genius of the designer. But it is altogether possible that the limit of the size of the locomotive would have been reached long ago if it had not been for the coming of the air-brake. This most important assurance of the safety of the railroad passenger came into its being in 1869, when George Westinghouse, its inventor, was permitted to try it on a Panhandle train. From the beginning of railroads the necessity for brakes was apparent, and in 1833 Robert Stephenson patented a steam brake for the driving-wheels. That same brake, with compressed air substituted for steam, is essentially the Westinghouse device of to-day. But Westinghouse made the air do the work of steam. After he had developed the idea he offered it to leading Eastern railroads, but they one and all declined it.