1886.] 



On the Sympathetic Vibrations of Jets, 



373 



brane is introduced into its path close to the orifice, scarcely any 

 sound can be heard in the ear-piece. But af the membrane be moved 

 away from the orifice along the path of the jet the sounds become 

 gradually louder, until at a certain distance (which varies both with 

 the character of the orifice and the intensity of the impressed vibra- 

 tions) a position of maximum purity and loudness is reached. At 

 greater distances the reproduction by the jet becomes at first rattling 

 and harsh, and finally unintelligible. In the latter case the jet will 

 be seen to break above the membrane. 



From this experiment we may draw the conclusions previously 

 arrived at for air-jets ; viz., that all changes produced by sound at the 

 orifice grow in accordance with the same law ; and that all changes 

 travel with the same velocity, which is probably the mean velocity of 

 the stream. 



The mode in which the jet acts upon the membrane becomes 

 apparent when instantaneous shadow photographs of vibrating jets 

 are examined. When the jet is steady, and the orifice strictly 

 circular and well insulated, the outline in the upper part of the stream 

 is that of a slightly conical rod, the base of the cone being at the 

 orifice. When, however, vibrations are impressed upon the support, 

 swellings and constrictions appear on the surface of the rod, which 

 become more pronounced as the fluid travels downwards. At the 

 breaking point the constrictions give way, those due to the more 

 energetic sound impulses being the first to break. When the im- 

 pressed vibrations are complex, the outline of the jet may be very 

 complicated. When the membrane is interposed, we have then a 

 constantly changing mass of liquid hurled against it, and vibratory 

 movements are therefore excited in it, proportional to the varying 

 cross section of the jet at its surface. 



It would appear at first sight that the mode of growth of the 

 vibratory changes in a liquid jet must be different from that which 

 characterises the vibrations of an air-jet. It is possible, however, by 

 special arrangements, to receive the impact of only a small section of 

 a vibrating liquid jet, and thus to get a reproduction of its vibrations 

 as sound. We are thus led to conclude that the sound effects of a 

 vibrating liquid jet may not be simply due to its varying cross section, 

 since actual changes occur in the translation- or rotation-velocity of 

 its particles. Experiment shows that these changes are greatest 

 along the axis of the jet. 



One of the most interesting and beautiful methods of studying the 

 vibrations of a jet consists in placing some portion of it in circuit with 

 a battery and telephone, whereby its vibrations become audible in the 

 telephone. A number of forms of apparatus for this purpose have 

 been constructed, but one will serve as a type. Savart in the course 

 •of his experiments showed that the vibrations of the jet are preserved 



