400 
MR. C. A. BELL ON THE SYMPATHETIC VIBRATIONS OF JETS. 
allows the distance of the membrane from the jet orifice to be easily varied. The 
receiving surface ought to have a motion of at least 8 or 10 inches. 
The water supply can be drawn from any kind of reservoir, capable of being 
fixed at different heights, and connected with the jet tube by soft black rubber¬ 
tubing. But water may also be drawn from the mains, when the pressure in the 
latter is tolerably constant. In crowded or noisy neighbourhoods a jet so supplied 
is exposed to many accidental vibrations ; but these vibrations, as well as the 
disturbances always produced when the pressure is regulated by an ordinary tap, 
may be deadened by connecting the latter with the jet tube through a coiled 
rubber tube immersed in a vessel of water. A narrow vertical tube or a mercury 
gauge connected with the supply-pipe may serve to indicate the pressure. 
Glass jet tubes are easily prepared in the way already described for air-jets ; but 
care should be taken to make the drawn-out ends as obtuse as possible. What is 
practically an orifice in a thin plate may be obtained by heating in a blow-pipe flame 
the squarely cut end of a tube of uniform thickness, with constant rotation, until it 
has contracted so as to form an orifice much smaller than required, and then, while the 
glass is still soft, blowing somewhat forcibly into the opposite end. 
Care should be taken to remove air-bubbles as completely as possible from the 
supply tube. A large bubble will frequently lodge in some bend at a distance from 
the orifice, and by its oscillation keep the jet violently disturbed. 
The fundamental properties of jets may now be illustrated as follows :—Having 
mounted in the sound-board a jet tube with an orifice about 7 mms. in diameter, let 
the reservoir be lowered so as to cause water to escape at a pressure of 15 cms. 
The jet will then form a continuous column for a short distance, beyond which it 
becomes troubled and breaks into drops. As the pressure is raised the length of 
the continuous column gradually increases, until the pressure approaches 160 cms., 
when this portion of the jet gradually loses its rod-like character, and becomes 
violently disturbed. Before this point is reached, if the orifice is not perfectly circular 
the surface of the continuous column will appear corrugated or beaded. 
Even at a pressure of 15 cms. the jet will sluggishly respond to ordinary voice 
sounds (which are always complex), or to taps on the sound-board or support ; but 
will remain quite indifferent to simple musical tones of moderate pitch, and to shrill 
or hissing noises. The visible effect of any disturbance of brief duration will be 
a momentary shortening of the continuous column, after which the jet resumes its 
normal appearance. If the source of disturbance be a continuous musical sound of 
sufficiently low pitch, the shortening of the clear column will persist as long as the 
sound lasts, and will bear some relation to its intensity. 
As the pressure is increased by raising the reservoir the range of tones to which 
the jet responds, and by which its continuous portion is shortened, is gradually 
extended, embracing notes of successively higher pitch ; while it is still affected by 
all the lower tones. At a pressure of about 45 cms. of water it will respond to 
