236 Mi C. Sondhauss on the Sounds 



produced in the middle of the water-surface, while the surface 

 is otherwise at rest; with greater pressures of 2 to 3 feet 

 rapid undulations are formed. No sound, however, is pro- 

 duced. If, however, a knife or edge of any kind be held in the 

 jet at a distance of a few millimetres from where it emerges, 

 tones are formed which are analogous to those of the current of 

 air; they are most certainly and most distinctly produced if a 

 plate with an aperture corresponding to that of the jet be held 

 over the jet. 



The apertures were mostly circular, yet four- or three-sided 

 apertures were also used. Large apertures require too great a 

 quantity of water, and the quantities of water which pass through 

 the upper aperture are liable to produce disturbing vibrations. 

 The best results were obtained with circular apertures 2 millms. in 

 diameter, which were therefore generally used for the sake of com- 

 parison ; apertures of 3*5 millims. and 5 millims. were also used. 



As the pressure of water increases, the tone is in general 

 higher and stronger ; but after it has been under this condition 

 for some time, it will suddenly become feebler, yet starts again 

 more powerfully when the pressure increases ; or it is quite silent, 

 and can only be again worked by a further definite increase of pres- 

 sure ; or all at once it will become deeper by a definite interval, for 

 instance a fourth, again attaining the same height or a greater 

 one as the pressure increases. If the pressure decreases, the 

 same phenomena occur in the inverse order, but with this 

 difference, that notes once formed continue for the most part, 

 even with a somewhat smaller pressure than that at which they 

 first occurred during the increase of pressure. With a definite 

 pressure of water some tones are formed particularly pure and 

 sonorous ; with others, the tones are accompanied by an un- 

 pleasant roaring and grating, which often occurs alone. Very 

 often three tones are heard at the same time, of which, when the 

 pressure is changed, one preponderates and is afterwards alone 

 heard. The mixed sounds, consisting of several notes which 

 frequently form no pure interval with each other, greatly 

 increase the difficulty of observing the height of the note, and 

 frequently make the observer distrust his own judgment. It is 

 often particularly difficult to decide to which octave the note 

 perceived belongs. 



The note is similarly altered by varying the distance of the 

 plate from the aperture ; by increasing the distance it gradually 

 becomes deeper, then suddenly becomes higher, or perhaps en- 

 tirely disappears, but occurs again at a still greater distance. 

 Even if the two apertures are somewhat different from each other, 

 by a suitable velocity and distance of the plates, the tones may 

 be evoked, though more feebly and to a less extent. With a 



