L. MECHANICS AND ENGINEERING. 471 



tions of the road and way affecting the movements of trains 

 or engines. 4. The passing of trains in every direction ; and, 

 5. The time and relation to such movements, etc., all in a 

 succinct form, upon the same roll or strip of paper. These 

 results are obtained by connecting with the reciprocating 

 parts of the points and signal-working, or with the interlock- 

 ing gear, a peculiar arrangement of electric contact making 

 and breaking apparatus, acting through a simple electro- 

 magnetic contrivance, which, in turn, operates on and de- 

 flects a pen or style, which records upon the strip of paper 

 the movements in question. 15 A, Sept. 5, 1874, 323. 



HISTORY OF DUPLEX TELEGRAPHY. 



In an investigation into the mathematical theory of the 

 workings of duplex telegraphy, the author, Mr. Schwendler, 

 electrical engineer of the Indian government, gives a short 

 sketch of the history of duplex telegraphy, in which he states 

 that as early as 1849 Messrs. Siemens & Halske, of Berlin, 

 took out a patent in England for the simultaneous transmis- 

 sion of a plurality of messages. In 1854 Dr. Gintl, of Vi- 

 enna, effected the practical solution of the same problem by 

 employing an electro-chemical method, and in the following 

 summer a differential method was independently arrived at 

 by Siemens & Halske, of Berlin, and by Frischen in 1855. In 

 Sweden Edlund employed a differential method, which he 

 had invented in 1848. The theory of Zantedeschi namely, 

 that of distinguishing electric currents passing simultane- 

 ously from opposite directions through the same conductor, 

 without in any way interfering with each other, and on 

 which that physicist has claimed the honor of having first 

 suggested the idea of duplex telegraphy is characterized by 

 Schwendler as being in direct opposition to the electrical 

 laws which were already known in 1829. None of the above 

 methods had any extended application ; they appear to have 

 been attempted doubtingly, and were generally rejected as 

 impracticable. Only recently, after a torpid existence of 

 almost twenty years, has duplex telegraphy secured the 

 amount of public interest it rightly deserves; and to Mr. 

 Stearns, of New York City, is due the credit of having ap- 

 preciated its value, and by means of his own inventions 

 proved its thorough practicability. The invention of the 



