s//t \\-ii.i.iA.\i XII-:MKNS % F.K.S. 143 



il. S-condly, by the annular form of the steam jet the 

 extent of surface contact between the air and the steam is greatly 

 increased, and the quantity of air delivered is by this means very 

 murh augmented in proportion to the quantity of steam employed ; 

 iilsn the great extent of surface contact tends to diminish eddies. 

 Thirdly, by discharging the combined current of steam and air 

 through the expanding parabolic delivery funnel of considerable 

 length, in which its velocity is gradually reduced and its momen- 

 tum accordingly utilised by being converted into pressure, the 

 degree of exhaustion or compression produced by the steam 

 jet is very materially increased under otherwise similar circum- 

 stances. 



The results of a long series of experiments with this form of 

 steam jet, both for exhausting and compressing air, have led to the 

 following conclusions : 



First, that the quantity of air delivered per minute by a 

 steam jet depends upon the extent of surface contact between 

 the air and the steam, irrespective of the steam pressure, up to 

 the limit of exhaustion or compression that the jet is capable of 

 producing. 



Second, that the maximum degree of vacuum or of pressure 

 attainable increases in direct proportion to the steam pressure 

 employed, other circumstances being similar. 



Third, that the quantity of air delivered per minute, within the 

 limits of effective action of the apparatus, is in inverse relation 

 to the weight of air acted upon ; and that a better result is there- 

 fore realised in exhausting air than in compressing it. 



Fourth, that the limits of air pressure attainable with a given 

 pressure of steam are the same in compressing and in exhausting, 

 within the limit of a perfect vacuum in the latter case. 



The principle of action of the steam jet had received but little 

 attention until the time of the interesting question raised in 1858 

 by the invention of the Gifl'ard injector, by means of which water 

 can be forced into a high-pressure boiler by a jet of steam of the 

 same pressure, or even of greatly inferior pressure. The physical 

 explanation of this remarkable fact, which was first attempted the 

 writer believes in a discussion of the subject in this Institution,* 



* See Proceedings of the Institution of Mechanical Engineers, January, 1860, 

 p. 39. 



