The instrument is not only regulated by the runner P, but also by a regulating screw V which allows the electro-magnet M to be properly adjusted. The regulating system is made more exact by the small screw V, and the instrument is connected with the line by the binding screw B. Of course this double arrangement is necessary for each of the sending systems. As I have already said, seven different messages might theoretically be sent at once in this way, but Mr. Gray has only adapted his instrument for four; he has, however, made use of the duplex system, which allows him to double the number of transmissions, so that eight messages may be sent at the same time, four in one direction, and four in another. Mr. Hoskins asserts that this system has been worked with complete success on the lines of the Western Union Telegraph Company, from Boston to New York, and from Chicago to Milwaukee. Since these experiments were made, fresh improvements have rendered it possible to send a much larger number of messages. Mr. Gray has also, aided by Mr. Hoskins, devised a system by which telephonic messages may be sent on a wire previously used for Morse instruments. Mr. Varley had already solved this problem, but Mr. Gray’s system seems to have produced important results, and has therefore a claim to our attention. We do not, however, describe it here, since it is not within the lines marked out for us, and those who are interested in the subject will find all the necessary details in a paper inserted in the ‘Journal of the Society of Telegraphic Engineers, London,’ vol. vi. _Mr. Varley’s System._--This system is evidently the earliest in date, since it was patented in 1870, and the patent describes the principle of most of the arrangements which have since been adopted by Messrs. Lacour, Gray, and Bell. It is based upon the use of his own musical telephone, which we have already described, but with some variations in its arrangement, which make it somewhat like the Reiss system.