chap, xx.) VENA CONTRACTA. 209 



The phenomenon of contraction is called the VENA 

 CONTRACTA, and its cause is represented in the diagram. 

 The molecules vertically above the centre of the 

 orifice stream straight clown and pass 

 out by the orifice, but the molecules 



at the side follow a curved course in v ;i;-v- 



the endeavour to get into the stream. 

 The direction of their motion can be 

 decomposed into the two elements, 

 one horizontal and the other vertical. 

 The horizontal components of op- 

 posite sides oppose one another. It 

 is thus evident that the molecules Fig. 100. Vena 

 not in line with the vertical of the 

 orifice oppose one another, and that they do this the 

 more, the farther they are removed from the vertical. 

 In consequence, the escape of fluid is opposed, and the 

 vena contracta formed. Owing to this delay, then, 

 the quantity of efflux does not reach the theoretical 

 amount. If, however, the diameter of the contracted 

 portion be taken as the diameter of the orifice, the 

 results are in harmony with the theory. The diameter 

 of the vena contracta ab is usually about two-thirds 

 that of the orifice. 



The normal quantity of efflux may be restored, 

 and the influence of the vena contracta counteracted, 

 by fitting a small tube to the orifice. If the tube 

 have a diameter equal to the orifice, and a length 

 two: or three times its diameter, the quantity dis- 

 charged in a limit of time is considerably increased. 

 The vena contracta is still formed, but the fluid, 

 expanding beyond it, reaches a greater diameter than 

 that of the jet, owing to the attraction exerted on the 

 fluid by the inner surface of the tube. 



Marriotte's bottle. --It is apparent, in the 

 case of a reservoir, that if the velocity of out- 

 flow is to remain uniform, the original level of the 

 o 7 



