518 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



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 ever}- particle near it. The result may be made clear 

 by the diagram (Fig. 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 



Pt A and B exert pressures in the direction M M and 



M \ j /' P N X X, those in the centre of the orifice in the direction 



R Q, those in the intermediate parts in the directions 



P Q, P Q. In consequence of the fluid in the space 



P Q, P 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 



Fie' 212 ^ ne 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 cylindrical 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 ma}' be increased to about 



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



tube, just as it would do if issuing 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 (Pig. 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 Bunsen's Alter 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 Liquid* Through Ricjid 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 the 



Fig. 213. 



