/A' WILLIAM SIEMENS, F.R.S. 141 



change of speed was the differential governor ; but short 

 of the differential governor the arrangement of the crossed-arm 

 governor with spiral spring appeared to him a very useful and 

 valuable one. 



ON A STEAM JET FOR EXHAUSTING AIR, &c., AND 

 SOME RESULTS OF ITS APPLICATION,* 



BY C. WILLIAM SIEMENS, Esq., D.C.L., F.R.S., Pres. Inst. M.E. 



THE steam jet, although it has long been used with such 

 remarkable success for producing a current of air to form an 

 artificial draught in locomotive and portable engine boilers, has 

 hitherto been extremely limited in its application to other pur- 

 poses ; for though its simplicity for the propulsion of air is a 

 great recommendation, the effect realised from a given expenditure 

 of steam has been extremely unsatisfactory, in comparison with 

 the results of the same steam in working an engine and air pump ; 

 nor has sufficient pressure of air been obtained from the steam jet 

 to render it applicable for pneumatic propulsion or the production 

 of furnace blast. The form and application of the steam jet 

 having remained hitherto essentially the same as in the original 

 steam blast of the locomotive in 1829, it occurred to the writer 

 that much might be done to improve its effect by a judicious 

 arrangement of the parts, so as to avoid eddies in the combined 

 current of steam and air, and also to utilise more completely the 

 initial momentum of the steam. In carrying out this idea, the 

 first results obtained about a year ago were sufficiently encourag- 

 ing ; and by gradually recognising more fully the points of essen- 

 tial importance, the writer succeeded in constructing a steam jet 

 exhausting apparatus capable of producing a vacuum of as much 

 as 24 inches of mercury, with steam of only three atmospheres 

 effective pressure ; and a useful effect has been obtained from the 



* Excerpt Minutes of Proceedings of the Institution of Mechanical Engineers, 

 1872, pp. 97-116. 



