18 



PHYSIOLOGY OP THE DOMESTIC ANIMALS. 



Fig. 212. 



motion through attraction, but also through their own mobility tend to 

 pass in an oblique line into the moving column. In fact, every particle 

 above the orifice endeavors to pass out of the vessel, and in so doing 

 exerts pressure on every particle near it. The result may be made clear 

 by the diagram (Pig. 212). 



Every particle above A B endeavors to pass out of the vessel, and 

 in so doing exerts a pressure on those near it. Those that issue near 

 A and B exert pressures in the direction M M and 

 N N, those in the centre of the orifice in the direction 

 II Q, those in the intermediate parts in the directions 

 B Qi B Q- I 11 consequence of the fluid in the space 

 B Q, B is unable to escape, and that which does escape, 

 instead of assuming a cylindrical form, contracts and 

 takes the form of a truncated cone. It is found that 

 the escaping jet continues to contract until at a distance 

 from the orifice about equal to the diameter of the orifice. 

 This part of the jet is called the vena contracta. It is found that the area 

 of its smallest section is about five-eighths or 0.62 of that of the orifice. 

 Accordingly, the actual value of the escape is only about 0.62 of its 

 theoretical amount. If a cjdindrical tube (termed ajutage), with a 

 length two or three times its diameter, be made the channel of exit of 

 the fluid, the amount discharged per second may be increased to about 

 0.82 of the theoretical amount. A contracted vein is formed within the 

 tube, just as it would do if issning freely into 

 the air; but from the adhesion of the water 

 to the interior of this tube, the section of the 

 column flowing from the tube is greater than 

 that of the contracted vein (Fig. 213). (The 

 contraction of the moving column of fluid 

 within the tube causes a partial vacuum, and if 

 a side tube, dipping into mercury, be connected 

 with the ajutage at this point the mercury will 

 rise in the vertical tube, demonstrating the 

 existence of the vacuum. This fact is made 

 use of in Bimsen's filter pump.) If a conical 

 tube be fitted to the orifice of exit, with the 

 smaller end in connection with the vessel, the efflux may be still further 

 increased, and fall very little short of the theoretical amount. 



Flow of Liquids Through Rigid Tubes. — If the ajutage inserted 

 in the side of the vessel has more than a certain length, the amount of 

 fluid escaping is very considerably reduced. This fact rests upon the 

 hydraulic friction produced between the moving liquid and the walls of 

 the tube in liquids which exert a certain amount of adhesion against tlie 



Fig. 213. 



