February i, 19 12] 



NATURE 



467 



•current u^ed produced a succession of sparks, each lasting 

 a less time, perhaps, than the fifty-thousandth of a second, 



Fici. 7.— Three photograp'i.«, separated by about one-fifiieth 

 of a Sfcond, of a vibrating vortex ring formed bj' the 

 union of two rings. 



•one, and only one, spark occurring at each alternation of 

 "the primary current, or aljout eighty per second. 



at the back of the camera, the arrival of the ring in the 

 field of view, and the occurrence of a spark, or, more pre- 

 cisely, of a brilliant flash of light. The timing was all 

 done by a falling weight, which made suitably timed 

 electrical contacts, one of which operated an electromagnet 

 to release the plate, and another the electromagnet of the 

 vortex ring gun. 



When a single ring first issues from the gun it is not 

 I. cognisable as a ring, but is surrounded by an ellipsoid 

 of coloured water, and is followed by a trail of colour. 

 l his is shown in the stereoscopic view in Fig. 2. As the 

 ring progresses, the colouring of this ellipsoid, in which 

 the ring is embedded, and the colouring of the trail, grows 

 paler and paler until the ring in the interior of the ellip- 

 soid alone is visible, and has the appearance shown in the 

 steroscopic view in Fig. 3. Fig. 4, which is of special 

 interest on account of the fortunate timing, shows a ring 

 which has already progressed half a metre or more, and 

 is just about to strike a silver watch-chain which hangs 

 suspended in the water. It should be noted that the chain 

 ha- begun to bend before an actual impact has occurred. 

 liiis, together with the other views, demonstrates that the 

 ling is at all times surrounded with an ellipsoid of water 

 \\ liich moves with the ring, and in its early stages is visible 

 as shown in Fig. 2. Water being a viscous fluid, the 

 material of this ellipsoid is gradually being drained off 

 and left behind as the trail, but also being as continuously 

 replenished with clear water, until it is invisible except 

 by its effects, which are made manifest in Fig. 4. 



The stereoscopic view in P'ig. 5 shows two rings which 

 have just issued simultaneously from two holes in the gun. 

 They are perhaps 5 or b cm. from the gun, and are already 

 seen to be approaching each other under the influence of 

 their mutual attraction. In the stereoscopic view in Fig. 6 

 two single rings have just united. .V careful study of this 

 |)i( uir • in a stereoscope will show just what has taken 

 place. Already the conditions are fully established for the 

 subsequent complicated vibratory motions of this type of 

 ring. The single view in Fig. 7 shows another ring at a 

 later stage, also made up of two separate rings, as it 

 appears in three successive stages separated by intervals of 

 about one-fiftieth of a second. Here is plainly seen the 

 four types of motion possessed by liquid vortices formed by 

 the union of two single rings. First, there is a rotation 

 about the vortex filament ; secondly, the forward motion of 

 the vortex as a whole ; thirdly, the motion of oscillation 

 of the extremities of the vertical diameter of the vortex 

 in a vertical plane lying parallel to the direction of for- 

 ward motion ; and, fourthly, a motion of oscillation of the 

 extremities of the horizontal diameter of the vortex in a 



I 



Fir,. 8. —Stereoscopic view of a vibratory ring in one phase of it* motion. 



lluuo events v\ii.- iiukI--, by the timing device, to occur 

 •simultaneously ; they were the arrival of the falling plate 



NO. 2205, VOL. 88] 



horizontal plane. These four inviiivM.T,, . a. cpt the second, 

 are too rapid to be observed satisfactorily with the eye. 



