138 Mr. Willis on the Pressure of a 



principle, their abscissae representing the distance of the gage from 

 the center, and their ordinates being taken proportional to (pres- 

 sure) X (distance), the ordinates becoming of course negative when 

 the pressure is less than that of the atmosphere. Hence the 

 area will represent the whole difference between the pressure on 

 the lower surface of the disc and the atmospheric pressure, the 

 upward ])ressure being jiroportional to the areas above the axis, 

 and the downward pressure to those below. These diagrams, 

 therefore, shew, by inspection, the variations of pressure at dif- 

 ferent distances of the disc from the plate. 



Thus at .5 it is plainly repelled, at about .25 in equilibrium, 

 from .24 to .024 attracted^, between .024 and .018 in equilibrium, 

 and from .018 to contact repelled. 



The center of the orifice is in all these figures at the left 

 hand, and the decimals are the distances of the disc from the plate. 



To ascertain the effect produced by varying the diameter of 

 the disc, a gage ABCD, Fig. 5, was provided, which terminated 

 in a flat horizontal tube, made so thin that it could be introduced 

 between two plates at a distance of .08 from each other, the 

 upper plate being, for the convenience of seeing the point of 

 the gage, of glass; and sustained by three little knobs or feet, 

 which served to maintain its distance and parallelism with the 

 lower plate, and yet allowed it to be moved about into any 

 position with respect to the orifice. 



Inferring then that the pressure of the current, estimated at 

 right angles to its direction, would be the same whether measured 

 in a direction parallel or perpendicular to the disc, the end of 

 the gage was placed, as in the figure, at right angles to the 

 radius, and upon moving it to different distances from the center 

 of the orifice, its indications were found actually to agree with 

 those already obtained by means of tlie gage before described. 



