144 



THE MECHANICS OF THE EARTH S ATMOSPHERE. 



If by alternately opening and closing- the stop-cock we allow two 

 droits to enter into the colorless liquid in rapid succession, then there 

 arises a ring formation for each droj) and the following one always 

 catches up with the preceding one. Different cases are then possible, 

 according to the differences of pressure that are used ; if these are 

 slight then the second ring is not able to penetrate the first one and a 

 formation, as shown in Fig. 14, remains for a long time visible in the 

 fluid. With greater differences of pressure, on the other hand, ring 

 No. 2 passes through ring No. 1, since the former contracts while the 

 latter expands. One can then observe that afterwards ring No. 1 en- 

 deavors on its part to pass through ring No. 2. Bat generally the 

 living force is by this time consumed, so that ordinarily the two rings 

 settle into the formation shown in Fig. 15. This interchanging passage 



^ 



2 



T 



2r 



Fig. ]4. Fig. 15. 



of the vortex rings through each other was predicted by Helmholtz 

 from theory in the memoir above referred to. 



Reusch has occui)ied himself experimentally with the formation of 

 vortex rings.* After having described in detail the formation of smoke 

 rings in the air, he passes to the formation of rings by the sudden en- 

 trance of a small quantity of colored liquid into colorless liquid. Al- 

 though in his arrangement of the experiments the transition of the 

 progressive into the vortex motion is very rapidly completed, still he 

 also has frequently observed the intermediate stages shown in Figs. 11 

 and 12 and described them very appropriately as mushroom formations. 

 The manner of this transition is seen directly from the examination of 

 Figs. 10 to 13. Evidently there arise two currents in the quiescent 

 liquid. The one current, indicated by the arrows A and 5, is produced 

 by the progressive movement of the drop, which moves forward in the 

 liquid almost as a solid body. The other current, in the direction of 

 the arrows C and i>, is principally produced by viscosity. The forma- 

 tion of the spiral surface of rotation is finally the necessary consequence 

 of these two opposite currents. 



*Poggendorli''8 u4n«a7eH, vol. ex, pp. 309-316. 



