76 PHYSICS 
The air within a tube may also be put into standing vibrations by causing 
a current of air, flowing past the tube, to break against the edges of the 
opening, waves being thus produced which are reflected from the bottom, 
and interfere with those subsequently created. In narrow tubes the air 
may be set into standing vibrations by bringing the open end of the tube 
against the lower lip, and blowing into it obliquely against the edge. The 
tones will be deeper in proportion to the length of the tube, and inversely. 
The so-called Pan’s pipe is an illustration of this condition of things. 
Upon the principles just explained depends the construction of organ 
pipes, which are made principally of wood, in which case they are four- 
cornered, or of tin, when they are made cylindrical. Figs. 53 and 54 
represent the form of the wooden, and figs. 55, 56, and 57, that of tin pipes. 
Such a pipe consists of the foot or pedal, p, the labium or mouth-piece, }, b’, 
and the tube. The pedal is hollow and sharpened to a cone below, to place 
it in the sound-board from which the pipe receives the air, which is to pro- 
duce in it the vibrations of sound waves ; above the widest part of the pedal 
is placed a bridge, /, which contracts the opening to a very fine slit, and 
thus directs the entering column of air against the sharp edge of the labium. 
The pipes themselves are supplied with air by means of a pair of bellows, a 
very convenient apparatus for which is exhibited in p/. 19, fig. 58. Between 
the feet of a small table, ss’, is attached a bellows, set in operation by the 
foot-board, p, and forcing its wind into the superincumbent wind box, which 
sends it through the tube, 7 into the upper sounding-board, cc. As this 
wind box, by continued motion of the bellows, will soon become full, if little 
air is used, a lever connected with a valve in the wind box strikes against 
a pin attached to ff, and thus lets out the superfluous air. The rod, Zi’, 
serves to give greater pressure to the wind box where a sharper current is 
required. In the upper floor of the sound-board are several holes, oo’, 
generally twelve, in which pipes may be inserted. These holes are always 
closed with valves, which may be opened by a register at hh’, upon which 
the air can enter into the pipes and cause them to sound. With a teebler 
wind the same pipes give a lower, and with a stronger a higher tone. 
Not covered tubes alone, or those closed at the upper end, can be thus 
intonated, but also those open above, and in precisely the same manner. In 
these the short and narrow tubes will always give the higher tone. Another 
method of employing open tubes consists in generating hydrogen gas in the 
apparatus represented in fig. 91, letting it escape through a fine mouth- 
piece, and after setting it on fire, placing a tube, ab, over the jet. 
Standing vibrations arise in an open tube, from the circumstance that a 
greater condensation takes place in the middle, the particles of air not being 
able to escape; as soon as this condensed portion comes to the open end 
of the tube, the particles expand, thus producing a rarefaction, which, sent 
back, traverses the tube in the opposite direction. As, however, at the 
open end a condensation and a rarefaction coincide, no vibration nodes can 
here occur, these necessarily existing in the inner portions of the tube ; if, 
therefore, the deepest tone of an open tube is to be equal to that of a closed, 
the former must be twice the length of the latter. 
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