Prof, Magnus o?i the Motion of Fluids. 9 



motion is imparted to the fluid, and in this case the air is ob- 

 served to enter sometimes with extraordinary violence. A fun- 

 nel-shaped whirlpool is seen to form round the jet, through 

 which the air passes downwards with a rotatory motion. This 

 is observed when the jet issues under a pressure of a column of 

 water 10 feet high, and is received into a vessel 2 feet wide. 

 From this it appears to me that there can be no doubt as to the 

 correctness of the explanation given in § 19 with regard to the 

 manner in which the air penetrates the fluid. 



23. "While observing the motion which ensues when one fluid 

 pours into another of a similar nature through an orifice placed 

 at some distance beneath the surface of the latter, it struck me 

 as being desirable first of all to ascertain experimentally whether 

 the action which the jet exerts upon a body capable of offering- 

 resistance varies with the distance from the orifice. I have car- 

 ried out these experiments by bringing a plate under the surface 

 of the water perpendicular to the direction of the jet, and mea- 

 suring the force with which the jet tended to move the same. 



To ascertain how far it is necessary that the plates used 

 should be completely flat, experiments were first made in the 

 air ; and to render the result independent of the gravity of the 

 water, a horizontal stream was applied. 



24. In the middle of a balance-beam, FG, fig. 10, a vertical 

 bar of metal was screwed ; and on this the plate cd was fastened, 

 so as to be perpendicular to the direction of the jet. When the 

 beam stood horizontal, and the jet was directed against the 

 middle of the plate, the latter was pressed back. By laying on 

 weights, the balance could be brought into its position of equili- 

 brium, and hence could be estimated the amount of the resist- 

 ance presented by the plate. 



Two perfectly smooth plates, the one of 9 millims. and the 

 other of 24 millims. diameter, were now fastened one after the 

 other at b, and a horizontal jet directed through the open air 

 against the centre. These plates were then exchanged for con- 

 cave hemispheres, the diameters of which were exactly equal to 

 those of the plates, and the force exerted was measured each 

 time. With a vertical pressure of 2 metres, and a distance of 

 100 millims. from the orifice/ made in a thin sheet of tin, and 

 having a diameter of 3 millims., the following results were ob- 

 tained. For — 



The plate. The hemisphere. 



of 9 millims. diameter . 22 grras. 42 grms. 



of 24 millims. diameter . 23 grms. " 38 grms. 



25. This greater pressure in the case of the hemispheres is 

 evidently due to the fact, that the water of the jet which strikes 

 them in the middle moves sidewards, and in this way exerts a 



