ETHER AND PONDERABLE MATTER 



revolve when the string is rolled between the fingers) 

 must, in a frictionless medium, retain its form and 

 whirl on with undiminished speed forever. 



While these theoretical calculations of Helmholtz 

 were fresh in his mind, Lord Kelvin (then Sir William 

 Thomson) was shown by Professor P. G. Tait, of Edin- 

 burgh, an apparatus constructed for the purpose of 

 creating vortex rings in air. The apparatus, which 

 any one may duplicate, consisted simply of a box with 

 a hole bored in one side, and a piece of canvas stretched 

 across the opposite side in lieu of boards. Fumes of 

 chloride of ammonia are generated within the box, 

 merely to render the air visible. By tapping with the 

 hand on the canvas side of the box, vortex rings of the 

 clouded air are driven out, precisely similar in appear- 

 ance to those smoke-rings which some expert tobacco- 

 sm< >kers can produce by tapping on their cheeks, or to 

 those larger ones which we sometimes see blown out 

 from the funnel of a locomotive. 



The advantage of Professor Tait's apparatus is its 

 manageableness and the certainty with which the de- 

 sired result can be produced. Before Lord Kelvin's in- 

 terested observation it threw out rings of various sizes, 

 which moved straight across the room at varying rates 

 of speed, according to the initial impulse, and which be- 

 haved very strangely when coming in contact with one 

 another. If, for example, a rapidly moving ring over- 

 took another moving in the same path, the one in ad- 

 vance seemed to pause, and to spread out its periphery 

 like an elastic band, while the pursuer seemed to con- 

 tract, till it actually slid through the orifice of the other, 

 after which each ring resumed its original size, and con- 



291 



