produced by a Fluid in Motion. 221 



III. 



When a fluid is flowing from an orifice in the bottom of a 

 vessel, it sinks steadily until within a short distance of the bot- 

 tom, when the particles of water immediately over the orifice 

 begin to move circularly around a centre, and a conical cavity is 

 formed extending from the water-surface to a greater or less 

 depth into the issuing stream. This cavity, according to M. 

 Laroque*, begins to be formed " quand la vitesse angulaire est 

 devenue suffisamment grande pour que la force centrifuge qu'elle 

 engendre puisse vaincre la pression hydrostatique et la cohesion 

 du liquide.-" 



If water possesses rotatory motion at the time of its efflux 

 from the bottom of a vessel, the conical cavity is formed much 

 sooner than it otherwise would be, and extends to a far greater 

 depth into the liquid. 



A stream of water entering water at right angles to its surface 

 does not produce rotation ; but if it enters at an angle differing 

 ever so little from a right angle, rotation in the direction in which 

 the stream flows is at once communicated to the water into which 

 it flows. 



A discharge-tube 2 inches long by ith of an inch diameter 

 was fitted into the bottom of a cylindrical vessel 10 inches high 

 by 4| inches diameter ; water was placed in the vessel until it 

 stood at a height of 9 inches from the upper orifice of the dis- 

 charge-tube; it was allowed to come to perfect rest, and the 

 orifice was then opened : the water sank steadily till within ith 

 of an inch of the bottom, when a cavity formed over the orifice. 



The same experiment was repeated ; but before the commence- 

 ment of efflux, rotatory motion was given to the water in the 

 vessel ; when efflux commenced, the conical cavity formed at 8 

 inches from the orifice of the discharge-tube and extended into 

 the issuing jet. When a discharge-tube of double diameter was 

 substituted, the cavity appeared at half an inch from the orifice of 

 the discharge-tube when the water was perfectly at rest prior to 

 efflux. When rotatory motion was given to the water, the cavity 

 appeared at 8 inches from the orifice and extended into the 

 issuing stream ; air entered through the cavity and expanded 

 the water as it issued from the vessel into an ellipsoid 3 inches in 

 its longer diameter by 1 inch in its shorter. The rate of efflux 

 was three times as slow as when no rotatory motion was given 

 to the water. 



If when the conical cavity has extended to some distance into 

 the issuing stream the latter is caused to enter water, the air 

 which enters through the cavity is seen to penetrate but a very 



* Ann. de Chim. et de Phys, March 1864, " Recherches Hydrauliques." 



