10 Prof. Magnus on the Motion of Fluids. 



further pressure against the spherical surface. That this is ac- 

 tually so, follows from the manner in which the water rebounds 

 from hemispheres of different diameters. While, for example, 

 the fluid from the 9 milHms. hemisphere returns in a direction 

 almost parallel with the original jet, that reflected from the 24 

 millims. hemisphere forms a curved surface of similar shape to 

 that shown in fig. 5, the longer diameter of which is somewhat 

 greater than that of the hemisphere. 



The reason is also plain whj^ the moving force is less in the 

 case of the large hemisphere than of the small one, although the 

 sui'face in the former case is greater. The smaller the radius of 

 curvature of the hemisphere, the greater will be that component 

 of the lateral pressure which exerts itself in the direction of the 

 jet. Only with perfectly flat plates do we obtain that portion 

 of the motion which acts exclusively in the direction of the jet ; 

 and hence, in ascertaining the quantity of motion, such plates 

 only have been used. 



26. The additional arrangement was as follows : — The level of 

 the water in the vessel was preserved constant by permitting 

 more to flow in at the top than escaped from the bottom. MN 

 (fig. 10) is a vertical tube bent to a right angle at M ; the inner 

 diameter thereof was 1 inch, and at it carried a collar of metal, 

 in the centre of which the orifice was situate. In all these ex- 

 periments the orifice was circular, and in most cases had a dia- 

 meter of 3 millims. The vessel ABCD, in which the horizontal 

 portion of the tube was immersed, opposite to the plate cd, mea- 

 sured 4 feet in length, 3 feet in width, and 16 inches in height. 

 It was quite filled with water ; and by an escape, c/h, which 

 reached to a certain height, the level of the surface was preserved 

 constant. The balance was so placed that it could slide along 

 the frame HI, and this frame was graduated so that the distance 

 of the plate from the orifice could be measured upon it. "Before 

 the vessel ABCD was filled with water, I convinced myself 

 that the jet which passed through the air struck the plate in its 

 centre. For the small distances here made use of, I imagined 

 that the direction of the jet might be regarded as horizontal, 

 particularly as it moved in a medium similar to itself. 



27. If in this experiment plates of different diameters be used, 

 and if the distance from the orifice remain constant, then the 

 force necessary to preserve the balance in equilibrium increases 

 with the magnitude of the plates up to a certain limit ; beyond 

 this the force repiains constant ; hence, by using tolerably large 

 plates, we were sure to obtain the entire effect. The smallest 

 plate which I used was 100 millims. in diameter. The following 

 table contains some of the results obtained when plates of differ- 

 ent diameters were applied. 



