.s/A' WILLIAM SIEMENS, F.R.S. 155 



delivery tube, was the measure of the working capacity of the 

 instrument. These were very distinctly marked characteristics of 

 a good steam jet ; and if they were attended to in the construc- 

 tion of the jet apparatus for any particular application, it would 

 be found that very economical results could be realised for the 

 steam expended. 



He exhibited a full-size specimen of one of the steam-jet ex- 

 hausters employed to exhaust the pneumatic tube connecting the 

 iclfirraph offices in London, so as to maintain a current of air 

 flowing continuously through the tube in the same direction. 

 Tin- tube formed a complete circuit, and the piston carriers, 

 whether put in at the commencing station of the circuit or at 

 any intermediate station, all travelled in the same direction ; 

 the result was that a tube of only 3 inches diameter was sufficient 

 for a great amount of work, because there might be ten or more 

 carriers in the tube at a time, all flowing towards the end station, 

 but capable of being intercepted and taken out at any intermediate 

 station. 



In answer to various Speakei-s 



The PRESIDENT replied that the expanding form of the delivery 

 tube from the steam jet was of considerable importance, though 

 not so much so in the case of air as where a denser fluid had to be 

 dealt with, as in the water jet of the Giffard injector. From 

 experiments that he had made with a parallel delivery tube, in 

 comparison with the expanding form, he had found that, in 

 exhausting a vessel to the extent of producing a vacuum of 

 '20 inches of mercury, the expanding tube rendered the jet about 

 10 per cent, more effective than it was with the parallel tube ; but 

 in exhausting to only a small extent, the greater density of the 

 fluid would cause the expanding tube to be as much as 20 per 

 cent, more effective than a parallel one. The form which he had 

 adopted for the expanding delivery tube of the steam jet was a 

 parabolic curve ; but the difference in effect between this and an 

 ordinary straight cone, with the same areas of passage at the two 

 extremities, would be immaterial. The mixing chamber, which 

 had to be of a definite length in proportion to the breadth of the 

 annular air passage, was made to diminish gradually in area, 



