156 THE SCIENTIFIC PAPERS OF 



somewhat in the form of a " vena contracta," the rate and extent 

 of contraction being determined in each individual case according 

 to the velocity of the combined issuing current. But the point of 

 chief importance in the instrument was the mode of bringing the 

 entering air into contact with the steam jet. As the working 

 power of the jet was proportionate to the extent of surface contact 

 between the steam and the air, the annular form of jet suggested 

 itself as the most suitable ; and the thickness of the annular film 

 of steam should be very small, about -gV^ 1 inch having been 

 found by experiment to be the maximum thickness of film that 

 was consistent with economical results. If the thickness of the 

 jet were increased beyond this amount, the particles of steam 

 inside the thickness of the jet would flow on at a greater rate 

 than the outer particles forming the skin of the jet. the latter 

 being retarded by contact with the air ; and eddies within the 

 atmosphere of the steam itself would be the result. Again, if the 

 steam jet were simply made to issue into a plain open tube, 

 through which the air had to be propelled, the result would be 

 very inconsiderable, because the eddies formed between the air and 

 the steam would then attain their maximum amount ; the steam 

 issuing with great velocity and the air being nearly stationary, the 

 latter would not be so much impelled in a steady current as set in 

 rotation and rolled forwards by the power represented by the 

 difference of velocity between the two fluids. It was consequently 

 requisite that the air should be accelerated as nearly to the full 

 velocity as practicable before coming into actual contact with the 

 steam ; and this was the most important point connected with the 

 apparatus. The area of the air passages was therefore gradually 

 contracted for some distance as they approached the issuing 

 orifices ; and the combined areas of the central air aperture inside 

 the steam jet and of the annular aperture outside the jet were 

 made to be together rather less than the area of the issuing orifice 

 at the outer extremity of the mixing chamber, through which the 

 combined current issued ; this arrangement ensured the air 

 attaining the full velocity of the combined jet, whereby the 

 amount of eddies and the consequent loss of power were reduced 

 to a minimum. 



The PRESIDENT considered that the suggested application of the 



