-Prof. R. Ball on Vortex-rings in Air. 1 3 



the smoke quite uninjured; it had not apparently left any of its 

 particles behind, nor had it admitted an atom of smoke into it ; 

 but it had drawn with it sufficient of the smoke to form a com- 

 plete shell, which enclosed it, and thus rendered the air visible. 

 The phenomenon is quite in accordance with that conservative 

 tendency which theory shows must belong to a vortex-filament. 

 The appearance is one of great beauty, and suggested the name 

 " negative smoke-ring." 



It was considered desirable to make these experiments before 

 an evening scientific meeting of the Royal Dublin Society. 

 Smoke-rings are seen to greatest perfection when directed along 

 a sunbeam admitted into an otherwise darkened room; and 

 though they can be seen beautifully in ordinary diffused sunlight, 

 by gaslight they make a very poor show ; special means of illu- 

 mination are therefore necessary. The following arrangement 

 is found to succeed admirably. By a simple optical contrivance 

 the radiation from a brilliant lime-light is concentrated into a 

 slightly conical beam, all other light in the room being extin- 

 guished. The box is placed in the beam, the orifice facing the 

 lamp, and from twenty to thirty feet distant from it ; at the box 

 the diameter of the circular section of the luminous cone is from 

 3 to 4 feet. A smoke-ring driven from the box will, if suitably 

 directed, traverse the beam through its entire length until it 

 reaches the lamp ; and as it is brilliantly illuminated throughout 

 its path, the appearance presented is of great beauty. A second 

 box can be placed at the lamp end of the beam, so as to show the 

 effect of the collision of rings. The fumes produced by burning 

 a small piece of phosphorus developes an ample supply of smoke 

 in the interior of the box. This mode of producing smoke was 

 suggested to me by Dr. William Barker ; it is simpler to manage 

 inside the box than the apparatus necessary for forming chloride 

 of ammonium. By the introduction of coloured glasses various 

 pleasing effects can be produced. If a sheet of tissue-paper be 

 very lightly attached to a frame and interposed in the beam, a 

 vigorously sent smoke-ring will sweep it away in a striking 

 manner. 



To exhibit the air-rings, the column of smoke already described 

 is placed so as to ascend through the beam, and rather nearer the 

 box than the lamp. An air-ring from the box is, of course, invisible 

 till it reaches the column, while in its passage from the column to 

 the lamp the curious phenomenon already mentioned is most beau- 

 tifully shown. The negative smoke-rings are much better seen 

 when thus illuminated than by ordinary daylight. A box of the 

 dimensions previously given, viz. a 2-foot cube with a hole of 

 8 inches diameter, was found to answer better for this purpose 

 than a somewhat smaller box with a hole of 6 inches aperture. 



