1871J Pneumatic Transmission. 307 



parcels was again revived, and it now seems likely to come into 

 very general use, especially in connection with the Post Office 

 and the Telegraphs. The want of some means of speedy 

 communication between the offices of the Electric and Inter- 

 national Telegraph Company in London, in addition to that 

 afforded by their lines of wire, probably led to the invention 

 of a pneumatic tube for that purpose by Mr. Latimer Clark, 

 and the first tube was laid down by that company in 1855. 

 This tube was of lead, and- the carrier (in which messages 

 were placed) nearly fitted the bore, and was covered with 

 felt. It turned out to be a complete success, and a similar 

 principle was adopted by the Prussian Telegraph Adminis- 

 tration in Berlin, between the Telegraph Office and Ex- 

 change, in the year 1863 ; in 1866 it was also adopted in 

 Paris in connection with the Electric Telegraph stations in 

 that city. Later still the principle has been introduced into 

 New York, and quite recently a line has been laid down by 

 Messrs. Siemens, in connection with the General Post Office 

 in London, for the conveyance of messages in original be- 

 tween Telegraph Street and Charing Cross and on to the 

 House of Commons, instead of sending them by Telegraph. 



There are two methods which have at different times been 

 adopted for impelling carriers through pneumatic tubes, the 

 one being by creating a vacuum in front of the carrier, which 

 is then impelled forward by the atmospheric pressure with 

 a force equal to the difference between the latter and the 

 vacuum. The other is by creating a " plenum" behind the 

 carrier, or, in other words, increasing the pressure behind 

 the carrier beyond that of the atmosphere, the difference be- 

 tween these two forces in pounds or ounces per square inch 

 representing the force expended in driving the carrier through 

 the tube. Medhurst, in his first invention, adopted the latter 

 principle, and the introduction of the vacuum for the pur- 

 pose is ascribed to John Vallance, who proposed it in a 

 pamphlet published by him in 1824. Experience has shown 

 that the vacuum is the far preferable manner of working 

 pneumatic tubes, and it is also more economical than work- 

 ing with a plenum. 



The difference between the effects of compression and ex- 

 haustion would appear, so far as recorded experiments upon 

 the subject show, to vary in the cases of tubes of different dia- 

 meters ; but as a general rule it has been observed that when 

 a carrier is inserted into a tube it is driven forwards with a 

 mean velocity corresponding to that with which the air at 

 the higher pressure is introduced behind it, or that at the 

 lower pressure is exhausted in front of it. In a paper read 



