SfK WILLIAM SIEMENS, F.R.S. 147 



advantage over an ordinary steam-engine and air-pump in first 

 cost and simplicity, and also in taking up much less space : the 

 latter advantage being of the greatest value when it is required to 

 work pneumatic despatch tubes in crowded localities where it is 

 di Hi cult to obtain room for steam machinery. A pneumatic tube 

 % inches in diameter, to be worked on the circuit system arranged 

 by the writer, has been laid down by the postal authorities in 

 London, from the Central Telegraph Station in Telegraph Street 

 to Charing Cross and back, with intermediate stations at the 

 General Post Office and near Temple Bar. The length of tube 

 forming the whole of this circuit is 6,890 yards, or nearly four 

 miles. This line was designed to be, and is, as a rule, worked by 

 means of an air-pump A, driven by a steam-engine, as indicated 

 in the diagram, Fig. 6, Plate 25. The pump draws air out of a 

 vacuum vessel V, and forces it into a pressure vessel P, both 

 vessels being in connection with the pneumatic tube T, as indi- 

 cated by the arrows. These vessels are introduced in order to 

 prevent the pulsations of the engine from being felt in the work- 

 ing of the tube, and are of course unnecessary when the tube is 

 worked by means of the steam jet. The piston carriers, which 

 are propelled through the tube by the vacuum produced, are of 

 the cylindrical form shown in Figs. 7 to 9, Plate 26 ; they consist 

 of a gutta-percha case covered with drugget or felt, and are made 

 an easy fit in the tube, so as to slide freely through it. 



This line has been experimentally worked by means of three 

 steam-jet exhausters, similar to the one shown in Fig. 2, arranged 

 in the manner shown at E E E, Fig. 6, so that all three draw air 

 out of the same tube F, the steam being supplied to them by the 

 steam pipe G. When working with these three exhausters, the 

 mean speed of a piston carrier travelling through the tube from 

 Charing Cross to Telegraph Street was 14| miles per hour. The 

 vacuum maintained in the tube was equivalent to 10 inches of 

 mercury, the steam pressure being 40 Ibs. per square inch ; and 

 the quantity of coal consumed under the boiler was 5G Ibs. per 

 hour. In the case of long lines of pneumatic tubes, it will be 

 better to work them with an exhauster at each end. 



For placing the piston carriers into the tube or taking them out 

 of it at the different stations S S, Fig. 6, without interrupting the 

 current of air, the intercepting apparatus shown in Figs. 10 to 12, 



L 2 



