It is realised that in order to keep the trainmen--and especially the engineers--alert and keenly alive to their work and responsibilities, it is necessary to make the periods of labour short; the same thing is found to apply to the machines also--they need rest to keep them perfectly fit. Before the engineer can run his train, the way must be cleared for him, and when the train starts out it becomes part of a vast system. Each part of this intricate system is affected by every other part, so each train must run according to schedule or disarrange the entire plan. [Illustration: TRACK TANK] Each train has its right-of-way over certain other trains, and the fastest train has the right-of-way over all others. If, for any reason, the fastest train is late, all others that might be in the way must wait till the flyer has passed. When anything of this sort occurs the whole plan has to be changed, and all trains have to be run on a new schedule that must be made up on the moment. The ideal train schedules, or those by which the systems are regularly governed, are charted out beforehand on a ruled sheet, as a ship's course is charted on a voyage, in the main office of the railroad. Each engineer and conductor is provided with a printed copy in the form of a table giving the time of departure and arrival at the different points. When the trains run on time it is all very simple, and the work of the despatcher, the man who keeps track of the trains, is easy. When, however, the system is disarranged by the failure of a train to keep to its schedule, the despatcher's work becomes most difficult. From long training the despatchers become perfectly familiar with every detail of the sections of road under their control, the position of every switch, each station, all curves, bridges, grades, and crossings. When a train is delayed and the system spoiled, it is the despatcher's duty to make up another one on the spot, and arrange by telegrams, which are repeated for fear of mistakes, for the holding of this train and the movement of others until the tangle is straightened out. This problem is particularly difficult when a road has but one track and trains moving in both directions have to run on the same pair of rails. It is on roads of this sort that most of the accidents occur. Almost if not quite all depends on the clear-headedness and quick-witted grasp of the despatchers and strict obedience to orders by the trainmen. To remove as much chance of error as possible, safety signalling methods have been devised to warn the engineer of danger ahead. Many modern railroads are divided into short sections or "blocks," each of which is presided over by a signal-tower. At the beginning of each block stand poles with projecting arms that are connected with the signal-tower by wires running over pulleys. There are generally two to each track in each block, and when both are slanting downward the engineer of the