Stream of Air upon a Plate. 



131 



parallelism with the lower plate, and keeping its center per- 

 pendicularly over the center of the orifice. It will be at first 

 violently repelled by a force which will be found to increase 

 till the disc reaches a point A-, thence the force diminishes to a 

 point I, where the disc appears in a state of unstable equilibrium. 

 Bringing it still lower it will be attracted by a force which 

 increases, reaches a maximum at m, and diminishes till the disc 

 is placed in stable equilibrium at n, and will be repelled if 

 pressed still further down. 



TABLE I. 



This table shews the distance of these points of stable and un- 

 stable equilibrium from the lower plate, in the case of an orifice 

 .25 in diameter, and a pressure of 6 inches. The measures 

 throughout this paper must be understood of inches, unless 

 othertvise expressed. The next table .shews the weight in grains 

 required to pull discs of various diameters off the lower plate. 

 It must be ob.served, however, that it is extremely difficult to 

 ascertain the exact weight required to do this, and this difficulty 

 is increased by a certain tremulous motion which the disc is apt 

 to acquire. The general results are, that when the diameter 

 of the disc is something less than twice that of the orifice, it is 

 blown off*, that upon increasing the diameter of the disc larger 



* This fact has been also observed by R. Younge, esq. in the Phil. Mag. April 1828, 

 p. 282. 



r2 



