154 THE SCIENTIFIC PAPERS OF 



the conditions of a steam jet he had been led to the conclusion 

 that, if the elastic force of the steam could be all employed for 

 giving velocity to its particles, and if each particle of steam so 

 accelerated could be brought into direct and immediate contact 

 with the air to be impelled, an average speed of the combined 

 current could be obtained, which should represent nearly the 

 whole of the elastic force originally existing in the steam. If this 

 combined current were then sent through a long expanding mouth- 

 piece or tube, enlarging gradually in a parabolic curve, so that the 

 velocity of the current should be gradually converted into pressure 

 by bringing the particles nearly to rest, it had appeared to him 

 that very favourable results might be obtained ; and the experi- 

 ments he had made had proved that these anticipations had been 

 correct. 



There were some remarkable results connected with this steam 

 jet. The quantity of work done by it, measured by the weight of 

 air delivered per minute, was proportionate absolutely to the 

 amount of surface contact between the steam jet and the air to be 

 impelled, irrespective of the pressure of steam employed. Thus in 

 discharging the combined current into the atmosphere, the same 

 weight of air per minute would be thrown out with a steam 

 pressure of 5 Ibs. as with GO Ibs., the only difference being that 

 with the lower pressure of steam the limit of exhaustion or com- 

 pression would be sooner reached than with the higher. The 

 experiments also showed that the degree of exhaustion or com- 

 pression capable of being produced by the steam jet, that is the 

 difference of pressure between the air at the jet and that in the 

 vessel from which the jet was exhausting or into which it was 

 compressing the air, was exactly proportionate to the steam 

 pressure ; so that with 100 Ibs. steam above the atmosphere the 

 degree of exhaustion or compression attainable was double what it 

 was with 50 Ibs. steam. A third result was that, in exhausting 

 the air from any vessel by the steam jet, it took as long a period 

 of time to exhaust the first 1 Ib. of pressure, from the atmospheric 

 pressure of say 15 Ibs. down to 14 Ibs., as it did to reduce the 

 pressure 1 Ib. in any lower part of the scale, say from 8 Ibs. to 

 7 Ibs. ; this was explained by the consideration that the weight of 

 air, which had to be put into motion with such a velocity as would 

 force it through the contracted opening at the base of the expanding 



