192 Lord Rayleigh's Acoustical Observations. 



tened glass nozzles, and are of the same character as those 

 given by fish-tail burners. In fig. 2 the flat side is pre- 

 sented to the observer; in fig. 3 the sheet (if undisturbed) 

 would be seen edgeways. The complication arises, partly at 

 any rate, from the different degrees of sensitiveness of dif- 

 ferent parts of the sheet, from which it results that one 

 part reaches disruption and loses its periodicity, while an- 

 other is yet in the earlier stages of the transformation. In 

 figs. 2 and 3 the jet is under the influence of a vibration 

 sufficiently powerful to cause it to flare in a regular manner; 

 in figs. 4 and 5 the vibration is less powerful, and the trans- 

 formations stop short of the final stage. 



Influence of Viscosity, 



It has already been noticed that the notes appropriate to 

 water-jets are far graver than for air-jets from the same 

 nozzles. Moreover, the velocities suitable in the former case 

 are much less than in the latter. This difference relates not, 

 as might perhaps be at first supposed, to the greater density, 

 but to the smaller viscosity of the water, measured of course 

 kinematically. It is not difficult to see that the density, 

 presumed to be the same for the jet and surrounding fluid, 

 is immaterial, except of course in so far as a denser fluid 

 requires a greater pressure to give it an assigned velocity. 

 The influence of fluid viscosity upon these phenomena is ex- 

 plained in a former paper on the Stability or Instability of 

 certain Fluid Motions*; and the laws of dynamical similarity 

 with regard to fluid friction, laid down by Prof. Stokesf, 

 allow us to compare the behaviour of one fluid with another. 

 The dimensions of the kinematic coefficient of viscosity are 

 those of an area divided by a time. If we use the same 

 nozzle in both cases, we must keep the same standard of 

 length; and thus the times must be taken inversely, and the 

 velocities directly, as the coefficients of viscosity. In pass- 

 ing from air to water the pitch and velocity are to be re- 

 duced some ten times. But, in spite of the smaller velocity, 

 the water-jet will require the greater pressure behind it, inas- 

 much as the densities differ in a ratio exceeding 100 : 1. 



Guided by these considerations, I made experiments to try 

 whether the jets would behave differently in warm and cold 

 water. At temperatures respectively about 130° F. and 52° F., 

 the difference was found to be extremely well marked. " With 



* Math. Soe. Proc. Feb. 12, 1880. 



t Camb. Phil. Trans. 1850, " On the Effect of Internal Friction of Fluids 

 on the Motion of Pendulums," § 6. See also Helmholtz, Wied. Ann. 

 Bd. vii. p. 337 (1870), or Reprint, vol. i. p. 891. 



