The electro-gas apparatus is similar to the electro-pneumatic, except that it employs com pressed carbonic acid has instead of air. This gas when compressed is far more elastic than air, and works with a livelier action.
The electric motor mechanisms are con sidered by the most advanced signal engineers to be the best all around power devices. The one considerable difficulty in their use is the absolute necessity to keep them free from moisture. In spite of this drawback, not yet overcome, many miles of road are protected by them and with a high degree of satisfaction.
History.—The actual operation of the block signal system dates from 1839 when the Great Western Railway, of England, first telegraphed the arrival and departure of its trains from station to station for short dis tances near London. In 1841, separate instru ments were introduced for each direction of traffic, showing whether the sections were clear or blocked, and telegraph bells were installed. In 1842, Sir W. F. Cooke, an English engineer, published a series of suggestions of the block system entitled 'Telegraphic Railways' and laid down the following principles: «Every point of a line is a dangerous point, which ought to be covered by signals. The whole distance, consequently, ought to be divided into sections, and at the end as well as the begin ning of them, there ought to be a signal, by means of which the entrance to the section is opened to each train when we are sure that it is free.D The constantly increasing volume of busi ness on railroads necessitated improved means for securing safety in operation, and in 1851 the South astern Railway of England devised the method of signaling by electric bells, by sound only, without the use of needle instru ments. In 1854, separate instruments for each track, with three positions of the needle, were introduced, and later the bell code was used in connection therewith. In 1875, the Sykes system of block signaling was invented and brought into extensive use, not only in Eng land but also in the United States. In this
system the outdoor signals at a block station were controlled by the signalman at the station in advance, by means of electrically-operated locks, the locking apparatus at the two stations being connected by an electric circuit. This effected a decided check against mistakes or carelessness, for while each signalman oper ated his own signals, he could not operate them until they had been unlocked by the signalman in advance, on request of the first signalman.
The advance has been continually toward the point of reducing the human element in signaling to a minimum, the ultimate goal being the strictly automatic operation of all running signals. The selection by a railroad of any particular type of signal system depends primarily upon its cost to install. The auto matic is the most expensive to set up, but the least expensive to operate, and the ,trend is constantly toward expenditures which actually insure the road against heavy losses through accidents.
Out of the 220,801 miles of railroad track on which passenger trains were being operated in the United States on 1 Jan. 1916, 124,447 miles were protected by some form of block signals. Of this total, on 51,120 miles the sig nals were of the automatic type, and on 73,327 miles they were of nonautomatic types.
Bibliography. 'The Railroad Signal Dic tionary' (New York 1911) ; Elliott, W. H., 'The ABC of Railroad Signalling' (Chicago 1909) ; Latimer, J. B., 'Railway Signalling' ( Chicago 1908) ; Lewis, L. P., 'Railway Signal Engineering' (London 1912) ; Pigg, J., 'Sig nalling' (London n.d.) ; Scott, R., 'Automatic Block Signals' (New York 1908) ; Webb, W. L., 'Railroad Construction' (1913) ; and the annual reports of the Block Signal and Train Control Board of the Interstate Commerce Commission.