500 



SCIENCE. 



[Vol. VII., No. 174 



and reproducing all sounds whose pitch is below 

 that of the jet normal, as well as some above ; 

 and that the timbre or quality of the impressed 

 vibrations is also preserved, provided that the jet 

 is at such pressure as to be capable of readily 

 responding to all the overtones which • confer 

 this quality. 



Other essential conditions for perfect reproduc- 

 tion are, that the receiving-membrane should be 

 placed at such distance from the orifice that the 

 jet never breaks into drops above its surface, and 

 that it should be insulated as carefully as possible 

 from the orifice. 



In order to assist the action of aerial sound- 

 waves on the fluid, it is advisable to attach 

 the jet - tube rigidly to a pine sound - board 

 about three-eighths of an inch thick. The sur- 

 faces of the board should be free, otherwise it 

 may be supported in any way. The receiving- 

 membrane is formed by a piece of thin sheet- 

 rubber tied over the end of a brass tube about 

 three-eighths of an inch in internal diameter. A 

 wide flexible hearing-tube furnished with an ear- 

 piece is attached to the brass tube. The jet-tube 

 is connected with an elevated reservoir by an 

 india-rubber pipe (fig. 6). 



With an apparatus of this kind, and a tolerably 

 wide jet-tube having an orifice about 0.7 mm. in 

 diameter, a pressure of about 15 decimetres of 

 water is required to bring the jet into condition 

 to respond to all the tones and overtones of the 

 speaking voice (except hissing sounds) and those 

 employed in music. At a somewhat higher press- 

 ure it will reproduce hissing sounds. It is not 

 easy for an untrained ear to distinguish between 

 the disturbing sounds and their reproduction by 

 the jet, when both are within range of hearing. 

 Vibrations may, however, be conveyed to a jet 

 from a distance in a fairly satisfactory way by 

 attaching one end of a thin cord to the jet-sup- 

 port, and the other to the centre of a parchment 

 drum. The cord being stretched, an assistant 

 may speak, sing, or whistle to the distant drum. 

 Other devices for conveying vibrations from a 

 distance are described. 



Now, when the jet is disturbed in any way, and 

 the receiving -membrane is introduced into its 

 path close to the orifice, scarcely any sound can 

 be heard in the ear-piece ; but, if 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 vibrations) a position of maximum 

 purity and loudness is reached. At greater dis- 

 tances 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 con- 

 clusions 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 mem- 

 brane becomes apparent when instantaneous 

 shadow-photographs of vibrating jets are ex- 

 amined. 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 ap- 

 pear 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 impressed 

 vibrations are complex, the outline of the jet may 

 be very complicated. When the membrane is in- 

 terposed, we have then a constantly changing mass 

 of liquid hurled against it, and vibratory move- 

 ments 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 characterizes 

 the vibrations of an air-jet. It is possible, how- 

 ever, by special arrangements, to receive the im- 

 pact of only a small section of a vibrating liquid 

 jet, and thus to get a reproduction of its vibra- 

 tions 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 meth- 

 ods of studying the vibrations of a jet consists in 

 placing some portion of it in circuit with a bat- 

 tery and telephone, whereby its vibrations be- 

 come 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 in the ' nappe,' 

 or thin sheet of fluid formed when the jet strikes 

 normally on a small surface. So far, then, as 

 vibratory changes are concerned, the nappe has 

 all the properties of the main stream. Although 

 the diameter of this excessively thin film is about 

 the same whatever be the distance of the surface 



