190 Lord Rayleigh's Acoustical Observations. 



With the very acute sounds, to which alone the high- 

 pressure gas-flame (lighted at the burner) is sensitive, little 

 can be expected from the use of resonators. 



Jets of Coloured Liquid. 



In the hope of being able to make better observations upon 

 the transformations of unstable jets, I next had recourse to 

 coloured water issuing under water. In this form the expe- 

 riment is more manageable than in the case of smoke-jets, 

 which are difficult to light, and liable to be disturbed by the 

 slightest draught. Permanganate of potash was preferred as 

 a colouring agent, and the colour may be discharged by mix- 

 ing with the general mass of liquid a little acid ferrous sulphate. 

 The jets were usually projected downwards into a large beaker 

 or tank of glass, and were lighted from behind through a piece 

 of ground glass. 



The notes of maximum sensitiveness of these liquid jets 

 were found to be far graver than for smoke-jets or for flames. 

 Forks vibrating from 20 to 50 times per second appeared to 

 produce the maximum effect, to observe which it is only 

 necessary to bring the stalk of the fork into contact with the 

 table supporting the apparatus. The general behaviour of the 

 jet could be observed without stroboscopic appliances by causing 

 the liquid in the beaker to vibrate from side to side under the 

 action of gravity. The line of colour proceeding from the 

 nozzle is seen to become gradually more and more sinuous, 

 and a little further down presents the appearance of a rope 

 bent backwards and forwards upon itself. I have followed the 

 process of disintegration with gradually increasing frequencies 

 of vibrational disturbance from 1 or 2 per second up to about 

 24 per second, using electro-magnetic interrupters to send 

 intermittent currents through an electro-magnet which acted 

 upon a soft-iron armature attached to the nozzle. At each 

 stage the pressure at which the jet is supplied should be 

 adjusted so as to give the right degree of sensitiveness. If 

 the pressure be too great, the jet flares independently of the 

 imposed vibration, and the transformations become irregular : 

 in the contrary case the phenomena, though usually observ- 

 able, are not so well marked as when a suitable adjustment is 

 made. After a little practice it is possible to interpret pretty 

 well what is seen directly ; but in order to have before the 

 eye an image of what is really going on, we must have 

 recourse to intermittent vision. The best results are obtained 

 with two forks slightly out of tune, one of which is used to 

 effect the disintegration of the jet, and the other (by means of 

 perforated plates attached to its prongs) to give an intermit- 



