Lord Bayleigh's Acoustical Observations. 193 



a drawn-out glass nozzle, a pressure of l^inch was enough 

 with hot water to cause flaring, whereas perhaps 3^- inches 

 were necessary with the cold water. At one inch the jet 

 in cold water was dead, but in hot water was still quite 

 active "*. 



These experiments were resumed at Cambridge in April 

 and May 1880 by Mrs. Sidgwick, with use not only of hot 

 and cold water but also of mixtures of alcohol and water, 

 whose viscosity is known to be much greater than that of 

 water alone. In order to retard cooling ; and thus to diminish 

 convection-currents, the experimental beaker was placed 

 within a larger one, and supported at the rim only, so as to be 

 surrounded by a jacket of warm air. The liquid intended to 

 form the jet was placed in a narrow glass jar about 10 inches 

 high, and the head was adjusted by raising or lowering the 

 jar and by varying the amount of liquid. The communica- 

 tion between the two vessels was by a glass syphon, whose 

 lower end was drawn out so as to form a suitable nozzle of 

 about 4*o inch diameter (fig. 6). The transparent tube was 

 advantageous on account of the more ready detection of air- 

 bubbles, the presence of which, especially near the nozzle, is 

 a source of disturbance. The apparatus stood in front of a 

 window, supported on a stone table carried by the walls of 

 the building, and the sensitiveness of the jet was usually 

 tested by dropping upon the table a large nail through a 

 height of about 2 inches. Observations were made of the 

 greatest pressure that the jet would bear, in the absence of 

 external disturbance, without flaring before reaching the 

 bottom of the beaker, and also of the least pressure at which 

 the jet was sensitive. In the case of the mixture of alcohol 

 and. water in equal parts, a modified arrangement was necessary 

 in order to obtain sufficient head. 



With plain water the colour was given by permanganate of 

 potash, and was discharged, as soon as the jet was broken up, 

 by ferrous sulphate previously added to the liquid in the 

 beaker. In some of the more delicate experiments it was 

 found necessary to bring the densities to a more exact equality 

 by the addition of indifferent saline material to the jet, but in 

 most cases this precaution is superfluous. For the jets of 

 methylated alcohol and water, permanganate was found un- 

 suitable, and was replaced by soluble aniline blue. 



The following table will give an idea of the results of a 



* Laboratory Note-book, Jan. 20, 1880. Prof. Osborne Reynolds has 

 availed hirnself of differences of temperature in order to vary the viscosity, 

 in some recent important observations npon the cognate subject of the flow 

 of water in tubes (Proc. Roy. Soc. March 15, 1883). 



Phil. Mag. S. 5. Vol. 17. No. 105. March 1884. O 



