form of block signalling. In the more elaborate form there are at each block-station three signals--the distant, the home, and the starting. The signals are often electrically interlocked, from one station to another, in such a way that it is mechanically impossible for the operator at A to give a signal for a train to pass that station until the signal at B has been put to danger behind the preceding train. A B C ----------------------------- It is seen that no two trains can be in the same block and on the same track at the same time. If all run at a uniform speed, they will be kept just the length of a block apart. If No. 2 is faster than No. 1, it will arrive at B before No. 1 gets to C, but will have to wait there. The block system, therefore, while it gives security, does not always facilitate traffic. The longer the blocks the greater will be the delay to trains; but the shorter the blocks, the greater the cost of establishment, maintenance, and operation. Various systems have been contrived to have block signals displayed automatically by the passage of trains. This, if it can be done reliably, will do away with the wages of part of the operators, and will also eliminate the dangers arising from human carelessness. But there are very great objections to relying solely upon the automatic action of signals, and automatic block signals are little used except as auxiliary to a system employing operators also. So used, they are of decided advantage, as they make sure that a danger signal is set behind every train in spite of the operator, and that it cannot be again set to the all-clear position till the train has passed out of the block. All this is accomplished by electricity. Brakes, interlocking, and the apparatus of signalling have been considered at length because they are very much the most important of all the appliances which go to increase the safety of operating railroads. They act chiefly to prevent collisions, but often prevent or mitigate accidents from derailments and other causes. Of all train-accidents happening in the last sixteen years, over one-third have been from collisions, and more than one-half from derailments. * * * * * [Illustration: Crossing Gates worked by Mechanical Connection from the Cabin.]