February i, 19 12] 



NATURE 



46; 



is such that they might be expected to touch on their edges, 

 upon a close approach they bend out of a straight course 

 and pass one another without an encounter. 



When a ring is aimed to approach the surface of the 



water, it is upon reaching the surface reflected in a very 



beautiful manner. As the surface of the water is 



^^^- approached the upper edge of the ring gains velocity over 



^^^■the lower edge, the plane of the ring tilting in such a 



I 



motion in their own plane. The vibrations are such that 

 the vortex changes from an ellipse with its major axis 

 vertical to an ellipse with this axis horizontal. The vibra- 

 tions are almost too rapid to be followed distinctly with 

 the eye, but make an interesting sight when the ring is 

 observed from the end of the tank as it approaches the 

 eye. 



The most scientifically interesting property of water rings 



Fio. ^. — Stereoscopic view of a fully-formed vertex ring. 



manner as to maintain itself always at right angles to the 

 line of motion of the ring. If the angle between the 

 surface of the water and the line of approach of the ring 

 to the surface be as much as 22°, the ring is still reflected. 

 If this angle is much exceeded the ring bursts through the 

 surface with a spurt of water. 



Refraction was also observed. The lower half of the 

 tank was filled with a dense salt solution, and the upper 

 half with pure water. The ring in passing from the upper 



Fig. 4. — Vortex ring about to strike a watch-chain after 

 progressing about half a metre from the gun. 



to the lower layer, with a slight angle from the hori- 

 zontal, was deviated from its straight path. The laws of 

 both reflection and refraction were proved to be, approxi- 

 mately at least, those of light. 



Rings ejected from a very exactly circular hole are them- 

 selves circular, and advance through the water with no 

 other motion visible than that of progression. If, on the 

 other hand, the hole is piiiptical, they have a vibratory 



NO. 2205, VOL. 88] 



may be observed when the end of the gun is provided with 

 two holes, one above the other. In the experiments tried, 

 the two holes, each 085 cm. in diameter, were placed with 

 their centres separated 2-55 cm. The two rings, which 

 issue simultaneously from the two holes, begin to attract 

 each other the moment they leave the gun, and at a 

 distance from the gun of about 6 to 8 cm. they come 

 togetiier with great suddenness, uniting to form a single 

 ring of approximately twice the circumference of one of 

 them. Tiie rings so formed proceed with the same velocity 

 as a single ring until broken up by impact with the end 

 or side of tile tank. 'Ihis ring possesses vibratory motions 

 that are remarkable. Unlike the single ring, which issues 

 from an elliptical hole with vibrations in one piano only, a 

 ring which is formed by the union of two rings has a very 

 complicated vibratory motion in planes both norma! and 

 parallel to the direction of forward motion of the rings. 

 These motions will be better understood by a study of the 

 photographic views. 



If the surface of the water in the tank be covered with 

 a layer of kerosene oil from 5 to 10 cm. deep, an interest- 

 ing phenomenon may bt^ observed, which is rendered more 

 apparent when the oil is coloured a deep red with a dye 

 known to the trade as .Soudan III. When the gun is filled 

 with uncolourcd water, and is located a few centimetres 

 below the surface of the oil, and an invisible ring is pro- 

 jected at a suitable angl<> with the surface of the oil, it 

 enters the oil, and is instantly conycrted into an oil ring 

 which proceeds to the upper surface of the oil ; it is there 

 reflected and re-enters the water as a visible oil ring, 

 which proceeds with only slightly diminished velocity for 

 a metre or more through the water. After the ring breaks 

 up the oil rises to the surface of the water, and after the 

 surface has been quitted another ring may be produced, 

 and the process may be continued indefinitely. As the 

 water is uncolourcd, the illusion is produced of red oil 

 rings issuing from the oil without any apparent agency Tor 

 their production. 



Experiments were conducted which demonstrated that in 

 the case of two liquids of different densities— at least if 

 these two liquids are not miscibic — it is possible to project . 



