THE simultaneous transmission of two telegraphic messages in opposite directions upon one wire, now known by the name of duplex telegraphy, dates back from the year 1853. In that year Dr. Gintl, the director of state telegraphs in Austria, described a method by which this feat could be accomplished, and in July of the same year the method suggested by Gintl was tried between Prague and Vienna. An improvement on this method was suggested by a German electrician, Frischen, by Messrs. Siemens and Halske, of Berlin, and other workers at this subject. Nevertheless, owing to practical difficulties, the experiments were little more than interesting additions to our knowledge. So little hope, indeed, was there of the practical realisation of this important matter that, in a standard work on telegraphy, published in 1867, after describing the early methods of duplex telegraphy, the author remarks:—“Systems of telegraphing in opposite directions and of telegraphing in the same direction more than one message at a time must be looked upon as little more than feats in ‘intellectual gymnastics’ very beautiful in their way, but quite useless in a practical point of view.” Such assertions should teach all scientific writers the lesson of “hoping all things not impossible, believing all things not improbable” an attitude of mind which, Sir John Herschel remarks, should always characterise the natural philosopher, and which, in the present day, is certainly the safest one. Within six years of the publication of the foregoing statement duplex telegraphy was not only largely employed in actual telegraphy but its use on certain busy lines became absolutely indispensable. The change from theoretical to practical success is due to an American, Mr. J. B. Stearns, who in 1872 succeeded in overcoming the main obstacle in duplex telegraphy, namely, what is known as the static discharge from the line. This Stearns accomplished by using a “condenser”; and further he developed a system of “duplexing” the line similar to the principle of the Wheatstone bridge.