of the Motion of Fluids, S^-c. 395 



(5) A fourth kind of adjutage employed by Venturi, was 

 one whicli converged to the vena contractu, and then diverged 

 from it. The equation (/) shews that, as the velocity will 

 decrease in passing from the minimum section to the mouth of 

 the tube, the pressure must increase : and this experience con- 

 tirms, and gives a rate of increase nearly agreeing with what 

 would result from the theory, on the supposition that the velocity 

 varies inversely as the transverse section of the tube. In this 

 example the stream is diverging as it leaves the mouth, and the 

 velocity of that portion of it which is in immediate contact with 

 the air, is very nearly the same, and must consequently be less 

 than the velocity at every point in the interior of the stream, 

 within a short distance from the aperture. As there must be 

 a section a little distant from the aperture, at which the stream 

 ceases to be divergent, here the velocity at every point will be 

 the same, and the pressure equal to that of the atmosphere. 



This velocity will be y , '^ .. ; and as the maximum section 



1 — a" 



will be larger than the aperture, this will account for a greater 

 expenditure from a conical diverging tube, than from a cylin- 

 drical of the same aperture :— which Venturi found to be the 

 case. 



(6) The last example will serve to explain the phenomenon 

 observed by M. Hachette, of the attraction of a disc, oi)posed to 

 a stream of water, issuing from an opening in a plane surface. 

 When the disc, which we suppose to be circular as well as the 

 opening, is placed sufficiently near the surface for tlie fluid to 

 fill the interval between them, the stream may be divided into 

 any number of equal portions, similarly situated in regard to 

 the centre of the disc, each of which is divergent in passing 

 towards the edge. And as the fluid on entering the air, is subject 

 to the atmospheric pressure, the pressure of that part which is 



