208 PHYSIOLOGICAL PHYSK.S. [Chap. xx. 



The same law applies to an opening made in the 

 side of the vessel, but in this case the distance through 

 which the molecules fall is to be counted as the heighi 

 of the column of liquid from the centre of the 

 opening to the surface of the liquid. The fact that 

 the opening is in the side does not affect the result, 

 seeing that the pressure is transmitted equally in all 

 directions. Thus from an opening in the side of a 

 vessel the liquid molecules are projected with a velocity 

 determined by the height of the liquid column above 

 the level of the opening. The liquid so projected does 

 not pass horizontally outwards, but describes a para- 

 bolic curve, due to the downward force exerted upon 

 it by the action of gravity. 



It is to be observed that, according to this principle, 

 the velocity of efflux is independent of the nature of 

 the fluid. 



Experiment proves the law regarding the velocity 

 of efflux, but not immediately. For were the rule 

 rigidly true, the quantity of liquid that escapes in a 

 unit of time ought to be equal to the velocity of efflux 

 X the area of the orifice. * But experiment shows the 

 quantity of efflux to be only about '6 of this amount. 

 The reason of this, however, is speedily apparent. On 

 observing a now of water from a small orifice in the 

 bottom of a reservoir, the stream of water is found to 

 have the shape represented in the diagram (Fig. 100). 

 Immediately on leaving the orifice the stream begins to 

 contract, and at last reaches a maximum of contraction 

 at a distance from the orifice nearly equal to its dia- 

 meter. After that the liquid begins to divide into 

 diverging streams, and the streams into drops, owing to 

 the feeble cohesion between the molecules which form 

 the liquid permitting easy separation from one another. 



* The velocity, we have seen, is J%gh ; the area of the orifice is 

 the square of its radius x 3 '14159 ; expressed thus, -nr 2 (* = 

 3 14159). 



