OF COMPRESSED AIR. 215 



of the aperture was noted when the merciuy 

 first indicated a sHght degree of vacuum. From 

 this pomt the shde, and consequently the tube, 

 was drawn outward 1 •32nd of an inch, and the 

 height of the mercury indicating vacuum again 

 noted. In this way, by stages of l-32nd of an 

 inch each, the tube was drawn to the outer 

 edge or periphery of the valve, and the height 

 of the mercury noted at each stage. The dif- 

 ferent heights of the mercury in all these stages, 

 with the exact places of the tube at the times, 

 were then marked by dots on paper, and these 

 dots being connected by lines, we obtained the 

 curve represented in Fig. 3. In this diagram, g 

 shews the point at which a vacuum was first 

 indicated, and the line from g to A, represents 

 the increase of the degree of vacuum, until at 

 A it is 1 1 inches of mercury. From this point 

 the reduction of the degree of vacuum is seen 

 by the curve from h to i. The straight line Af, 

 a little lower down, represents the pressure which 

 the mercury shewed when the tube was over 

 the aperture. 



The valve was now raised higher from its seat, 

 and the tube moved as before, and data obtained 

 for the formation of other curves. When the 

 valve was 3-16ths above the seat, the tube being 

 placed over the aperture shewed a pressure of 



