Prof. Magnus on the Motion of Fluids. 19 



is equal to the quantity which during the same time passes 

 through a. From the difference of level, the diminution of the 

 pressure mentioned in § 35 may be estimated. 



38. If it so happen, that, after the fluid in the chamber HF 

 has sunk to the bottom of the tube at C, the difference of level 

 in both chambers is not yet sufficient so to diminish the mo^^ng 

 force that the quantity of water passing through a section of 

 the wide tube shall be equal to that passing through the small, 

 then the water in the tube experiences during its motion a less 

 pressure in the direction from A to B than from B to A, although 

 the static pressure, i. e. that which would be exerted were the 

 water tranquil, in the latter dnection is simply due to the atmo- 

 sphere, whereas in the former direction it is made up of the 

 atmospheric pressure and that due to the difference of level. In 

 consequence of this greater pressure at B, the air enters the tube 

 in bubbles, as shown by the experiment described in § 6. 



39. So also iu the experiment described § 7, where a thin jet 

 prevented the outflow of water through a wide tube ; the air en- 

 tered the water through that tube, and it continued to enter till 

 the fluid in the vessel A, fig. 8, attained such a height that the 

 pressui'es from both ends of the tube were equal. "When the 

 pressure from the interior predominates, the water flows out. 



40. If, while the water flows through the tube de, flg. 8, the 

 pressure of the air within the tube diminishes, as in the experi- 

 ment § 13, the air will penetrate the water, and be carried for- 

 ward by it until an equality of pressure from both sides is 

 established. 



41. The increased effect of a jet directed against a plate, as 

 shown in the experiments described § 27 to § 29, finds also, I 

 imagine, its explanation iu the fact, that the pressure of the 

 moving fluid is less than that of the tranquil. A fluid, for 

 instance, after striking a plate, moves, when the latter is large 

 enough, along its sm'face ; and this parallel motion along the 

 plate diminishes the pressure against it. Hence the pressure 

 against that side of the plate which faces the jet will be less than 

 that from behind, whei-e the fluid is motionless. This difference 

 of pressure will be greater according as the velocity of the fluid 

 and the size of the plate are increased. But the velocity is mani- 

 festly greater when the plate is near the orifice than when it is 

 placed at a distance from it ; because when the plate is near to 

 the orifice, the jet has had less time to widen, and consequently 

 the space through which the water lias to pass pai'allel to the 

 plate is increased. Hence the nearer the plate to the orifice the 

 greater is the difference of pressure before and behind, and con- 

 sequently the less will be the force necessary to cause the plate 

 to return to its position of equilibrium. This result manifests 



C2 



