18 Prof. Magnus on the Motion of Fluids. 



tion of velocity ; but as the moving force is constant, a greater 

 quantity of fluid mil always pass through a distant section in a 

 certain space of time than through a section situate nearer to 

 the orifice. 



As the pressure exerted by the fluid during its motion is less 

 than its pressure when motionless, it rnight be imagined that 

 our fundamental notion is incorrect, according to which no por- 

 tion of the moving force is destroyed ; for part of this force 

 seems necessary to balance the difi'erence of pressure in the case 

 of liquids, and in the case of gases to increase the density. 



But that this is not so follows from the fact, that the diminu- 

 tion of the pressure is caused by the forward portions of the fluid 

 passing through a cross section of the jet more quickly than those 

 which follow. If, therefore, the difiierence of pressure lessens 

 the velocity of the original mass, this very diiference sets the 

 fluid at each side in motion, and in this way the loss of force is 

 exactly compensated. 



From this it follows — 



(1.) That a jet which streams into a substance similar to itself, 

 sends more water in the same time through a cross section situ- 

 ate at a distance from the orifice than through a section sitiiate 

 nearer to the same. 



(3.) That, in consequence of this, the pressure of the fluid 

 when in motion is less than its pressure when in a state of rest. 



36. These two propositions help us to a very simple explana- 

 tion of the phsenomena described in the foregoing pages. In 

 the experiment of Venturi, cited in § 4, more water passes 

 through the cross section SM of the canal SMRV, fig. 1, than 

 passes in the same time through the tube CA. A portion of 

 the water contained in the vessel must therefore be carried for- 

 ward along with the stream. 



In like manner, in the experiment described § 5, more fluid 

 passes through the wide tube ABC, fig. 2, in the same time than 

 through the narrow tube at a ; and as a motion from the sides 

 cannot here take place, the fluid in the portion HF of the vessel 

 EDGF passes, in consequence of the diminished pressure through 

 the orifice BC, and hence the surface sinks. 



37. If, however, the fluid in the chamber HF sinks while that 

 in HE remains unaltered, the pressure due to the difference of 

 level operates against the motion of the jet, and a diminution of 

 the moving force is the consequence, so that it is less in the 

 cross section of the tube AB at A than in the narrow tube a. 

 With this decrease of force, the quantity of fluid which passes 

 A in the unit of time decreases also; the surface of the fluid 

 in the chamber HF will therefore sink until the quantity of 

 fluid which passes through the section at A in the unit of time 



