Lord Rayleigh's Acoustical Observations, 191 



tent view. The difference of frequencies should be about one 

 per second. When the means of obtaining uniform rotation 

 are at hand, a stroboscopic disk may be substituted for the 

 second fork. It was, in fact, with the use of such a disk, driven 

 by a water-engine, that the drawing (fig. 1, PI. IV.) was made 

 by Mrs. Sidgwick in August 1880. It is hardly necessary to say 

 that these appearances are difficult to reproduce in drawings, 

 and that the result must be regarded merely as giving a 

 general idea of what is actually observed. The upper part of 

 the jet is seen sufficiently steadily to be pretty accurately 

 copied ; but further down true periodicity is lost, and no 

 steady impression is produced upon the eye. 



The carrying out of these observations, especially when it 

 is desired to make a drawing, is difficult unless we can control 

 the plane of the bendings. In order to see the phases properly 

 it is necessary that the plane of bendings should be perpen- 

 dicular to the line of vision ; but with a symmetrical nozzle 

 this would occur only by accident. The difficulty may be got 

 over by slightly nicking the end of the drawn-out glass nozzle 

 at two opposite points. In this way the plane of bending is 

 usually rendered determinate, being that which includes the 

 nicks, so that by turning the nozzle round its axis the sinuo- 

 sities of the jet may be properly presented to the eye. 



Occasionally the jet appears to divide itself into two parts 

 imperfectly connected by a sort of sheet. This appears to 

 correspond to the duplication of flames and smoke-jets under 

 powerful sonorous action, and to be due to what we may regard 

 as the broken waves taking alternately different courses. 



Fish-tail Burners. 



" Experiments upon jets from fish-tail burners *. — As with 

 gas, so with smoke and coloured water, these are sensitive, 

 and when much excited throw out tall streamers in the per- 

 pendicular plane. I have not yet fully succeeded in tracing 

 the genesis of these, but believe them due to the rupture or 

 collision of the sinuosities w r hich are formed in the quickly- 

 moving part of the sheet. When the sheet, seen broadways 

 on, is excited by slow vibrations, a line of deepened colour is 

 seen to descend, and presently becomes very deep. This 

 means that the sheet is so far bent over as to be seen 

 tangentially." 



Even with the best arrangements as to sensitiveness and 



intermittent vision, the appearances presented by these jets 



are somewhat difficult to interpret and to reproduce in a 



drawing. The jets shown in figs. 2-5 issued from flat- 



* Laboratory Note-book, Dec. 12 ; 1879. 



