412 Mr. Challis on the general Equations 



(1) To exemplify this equation, let us take the instance of 

 air contained in any vessel, and driven from it through any 

 small orifice into the atmosphere, by being made subject to a 

 given pressure. Let IT be the given pressure, and let m^ be the 

 velocity where the pressure has this value. Then, neglecting the 

 etFect of gravity, 



J) 



from which, 2a' hyp. log. ^ =01,' -a.'; (4), 



hence the pressure i.s less as w is greater. 



This property will enable us to explain the phaenomenon, 

 lately exhibited before the Society by Mr. Willis, of the attrac- 

 tion of a flat plate, opposed to a stream of air issuing from an 

 orifice in a plane surface. Supposing the plate to be a circular 

 disc, the orifice also to be circular, and their centres to be at 

 a small distance from each other in a straight line, at right 

 angles to the planes of the surface and the plate, it is evident 

 that the stream may be divided into as many equal portions as 

 we please, having all the same motion, and related in the same 

 manner to the centre of the disc. Also if a circle be described 

 with the centre of the disc as centre, and radius equal to any 

 length r, the same quantity oi fluid must pass this circle in the 

 same time, whatever be r. 



Hence wpr = c, and p = — . 



Substituting this value of p in the equation above, and neglecting 

 (i>i, which in the experiment was very small, 



2a' 



a'c 



o>e — _ . 

 Ur 



