214 ON DIVERGING STREAMS 



obtained, it appeared that wiiile the valve ad- 

 hered to the seat, and remained at but a small 

 distance from it, a circular stripe or flat ring of 

 attenuated air was found between the valve and 

 its seat, and near to the aperture B, the air at 

 the same time in the parts further from the 

 aperture becoming more dense, until close to 

 the periphery it became nearly of common 

 atmospheric density ; but as the valve was raised, 

 the ring of attenuated air approached the outer 

 part or periphery of the valve. 



To find the form and nature of this ring, it 

 now appeared desirable that the different heights 

 of mercury in the same tube, indicating degrees 

 of vacuum should be ascertained at small and 

 equal distances, beginning at the edge of the 

 aperture, and proceeding along a radial line to 

 the periphery of the valve. To accomplish this, 

 a moveable slide was dovetailed into the valve, 

 and in this slide was inserted the lower limb of 

 one of the double syphon tubes with mercury 

 in it as before, as seen in Fig. 2, where the tube 

 is placed over the aperture, and shews a pressure 

 from the compressed air of li inches of mercury. 



This valve being placed on the seat, the slide 

 j\ f, was moved until the tube came over the 

 seat, and the distance of the tube from the edge 



