Stream of Air upon a Plate, 133 



FG a circular piece which performs the office of the disc, 

 and is furnished with three legs of slender wire rs, rs, ts, 

 passing through holes in the lower plate, and by which it is kept 

 in its place with its center coinciding with the axis of the tube, 

 while its distance from the lower plate is determined either by 

 the three screws t, t, t, which rest on the lower plate, or by 

 placing washers of known thickness on the wires rs. The 

 diameter of its lower surface is 2.4 inches. 



This disc is perforated with a circular aperture vwx (Fig. 3.), 

 into which is accurately fitted a plug H furnished with a shoulder 

 pp, which rests on the upper surface of the disc, and serves to 

 keep the lower surfaces of FG and H in the same plane, and these 

 lower surfaces being turned flat in a lathe, both from the center 

 of the plug i/, it is evident that they will accurately coincide 

 in whatever position H is turned. On the top of H is fixed a 

 water-gage fi^LM which communicates with a small hole i (diameter 

 .05) drilled in the lower surface of H. The relative positions of 

 FG, H and i, will be better understood by referring to Fig. 3, 

 which is a view of the lower surface of the disc. It will be 

 evident that this hole i may be placed at any required distance 

 from the center FG, while the gage will measure the difference 

 between the atmospheric pi'essure and the pressure at that point 

 of the lower .surface arising from the blast. An index z, traversing 

 a graduated scale, serves to shew the distance of i from the center 

 of FG. 



It appears from this instrument that in general the pressure 

 on the disc will be such as is represented in Fig. 4, where the 

 dark parts represent condensation, the mean tint the pressure 

 of the atmosphere, and the light parts rarefaction. 



Proceeding from the center of the disc we come to a circle 

 Aa, on every point of who.se circumference the pressure equals 

 that of the atmosphere, then to a circle Bb, where rarefaction is 



