of the Motion of Fluids, Sfc. 415 



which occasions another maximum of density, the position of 

 which depends on the energy of this cause. 



(2) When air, subject to a given pressure IT, is made to 

 issue through a small orifice into the atmosphere, the equation, 



Sfl^hyp. log. ^ =a,f-a>«, 



applicable to this case, shews that the points of equal velocity 

 are also points of equal pressure: consequently, that as the 

 pressure at the surface of the stream must be nearly the atmo- 

 spheric pressure, if the stream contract like water* there will 

 be a converging surface at every point of which the velocity is 

 nearly the same, and greater than the velocity at all points 

 within it, so that the pressure within the surface will be greater 

 than the pressure of the atmosphere. If a tube be fitted to the 

 orifice, exactly of the shape of this surface, the pressure on every 

 point of it will be that of the atmosphere : if the tube have more 

 convergence than corresponds to the natural contraction of the 

 stream, it will be pressed from within to without. Conceive the 

 stream to pass between two plane elastic laminae converging 

 towards each other, and let their inclination be greater than 

 that which the nature of the stream requires. Their extreme 

 edges will then be pressed outward. By this means a small 

 portion of the stream will become divergent ; and if w, = the 

 velocity where the pressure is P, the atmospheric pressure, it 

 will be seen by the equation 



2 a' hyp. log. ^ = «./ - W-. 



* The foregoing example makes this probable. See also Dr. Young's Lectures on 

 Natural Philosophy, Vol. II. p. 534. 



Vol. III. Part III. 3 H 



