We have seen that Mr. Clark designed the first tube used in connection with the telegraph, and that it was a single tube, operated in one direction only by vacuum, being operated only when there were messages to send. This was extended and improved by Mr. Varley, who increased the diameter of the tubes from one and one-half inches to two and one-quarter inches, and operated them in both directions, using vacuum for sending in one direction and compressed air for sending in the other. The air current was maintained in the tubes only when messages were sent. Great credit is due to Sir Rowland Hill, who, in 1855, had a proposed method of conveying mails in the city of London, through nine- and thirteen-inch tubes, thoroughly investigated. It was decided at this time that the saving in time over that consumed by mail carts would not warrant the expense of installing such a system. =The Siemens Circuit System.=—The next progressive step was made by Siemens Brothers, of Berlin, who proposed a new system called the “circuit system,” in which two tubes were used, the up tube being connected to the down tube at the distant end. The air was compressed into one end of the circuit and exhausted from the other, and, furthermore, it was kept in constant circulation. Carriers were despatched by inserting them into the air-current without stopping it, in one direction in one tube or in the opposite direction in the other. Another feature of the Siemens system was the placing of three or more stations on one double line of tubes. Carriers could be stopped at an intermediate station by inserting in the tube an obstructing screen which the air would pass but which would stop a carrier. This system is described in detail in a paper read by Mr. Carl Siemens before the Institution of Civil Engineers, London, November 14, 1871, Vol. XXXIII. of the Proceedings. The Siemens apparatus for sending and receiving carriers consisted of two short sections of tube attached to a rocking frame so that either could be swung by hand into line with the main tube. One of the tube sections was open at both ends, and was used for despatching carriers. A carrier was placed in it, then it was swung into line with the main tube, when the air-current passing through swept the carrier along. The other tube section contained a perforated screen in one end and was used to receive carriers. When it was in line with the main tube and a carrier arrived, the carrier was stopped by striking the screen, then the tube section was swung to one side and the carrier pushed out with a rod. A by-pass was provided for the air around the apparatus so that its flow was not checked when the tube section was swung. When a carrier was despatched to an intermediate station, a signal was sent, and then the section of tube containing the screen was interposed in the line of the tube to stop the carrier upon its arrival. The carriers used by Mr. Siemens were made of