314 Pneumatic Transmission. [July, 



equal differences of 9 inches of mercury above and below at- 

 mospheric pressure, the transit time of a carrier the whole 

 distance of 3000 feet was 95 seconds from the station to the 

 Exchange, whilst it only occupied 70 seconds in returning. 

 The pressures now employed are, in the one reservoir, 7 inches 

 of mercury over, and in the other 6 inches under atmo- 

 spheric pressure. With this arrangement the transit times 

 were, from station to Exchange 1 min. 30 sec. ; and from 

 Exchange to station 1 min. 20 sec. 



In the system in use in Paris the same apparatus consti- 

 tutes both the receptor and transmitter for the carriers. It 

 consists of a cast-iron stand or pedestal, surmounted by an 

 air-tight box, in front of which is a lid or door. Two tubes 

 enter this box from opposite sides ; one leading to the pres- 

 sure reservoirs and communicating through a cock or valve 

 with a branch below it with the box, whilst the other branch 

 from the box is connected with a tube open to the air at the 

 top, and also provided with a cock. A central vertical tube 

 closed at the top is used when carriers arrive, and acts then 

 as an air buffer, against which they expand their force. Be- 

 neath the box another tube leads to the next station. In 

 sending a message it is placed in a box and the door shut, 

 the cock communicating with the compression reservoirs is 

 then opened, and the pressure of air blows the carrier 

 through the tube. At this time the cock communicating 

 with the open tube is kept closed, but when a message is to 

 be received this cock is opened and the other kept closed — 

 the open tube admitting the escape of the air in front of the 

 carrier. 



According to Mr. Siemens's new method, a complete circuit 

 is formed by the current of transmission, with which several 

 stations may be brought into communication with each 

 other. The transmitting and receiving apparatus is ex- 

 tremely simple, and consists of two short pieces of tubing 

 the same diameter as the main tube, and out of the latter a 

 piece is removed of equal length. By means of a crank, or 

 rocking shaft movement, either of these short tube pieces 

 may be connected with the main tube by a simple movement 

 of a lever, and thus brought into circuit. One of these short 

 tubes is open throughout. This is the transmitter. It is 

 ordinarily kept in circuit, so that messages to other stations 

 beyond may pass through. When it is desired to send a 

 message the circuit is broken by moving the transmitter a 

 little to one side ; the carrier with its message is then placed 

 inside, and after communicating by signal with the station 

 for which it is intended, the transmitting tube is once more 



