44 JOURNAL AND PROCEEDINGS. 
consider two different hypotheses, we will confine our attentions to 
the results of the investigations of the most renowned scientists who 
have worked on the subject. 
Let us, at the outset, establish a distinction between musical 
tone and mere noise. Noise is sound in which the vibrations are 
irregular and uneven ; tone, that in which the vibrations are regular 
and uniform. 
Air-vibrations can be started by the vibrations of (a) a string, 
(b) a metal or wooden tongue, and (c) a jet of wind leading to the 
air contained in a pipe open at one end or at both. 
To illustrate the vibrations, consider a row of boys standing one 
behind another ; if some one pushes the last one, that sends a 
pressure from him to the next, and on to the next and the next and 
so on until it reaches the last one, who, having no one else to push, 
goes over. As each push goes on to the next boy there will be an 
almost imperceptible pull-back or reaction, just as a pendulum 
pushed to one side will swing back past the stationary point. The 
push of the boys we call a “condensation” or closing up, which 
leaves behind it an “expansion” or stretching, which will be seen 
in the action of the air inside an organ-pipe. As soon as the wave 
of condensation has passed out at the end of the pipe the reaction 
behind it has prepared the way for a new wave to begin. To con- 
sider the row of boys again: If the boy at the end of the row is 
facing a wall, he will not fall over, but will push at the wall; thus a 
reflex wave of push will be sent back along the line. So, if the 
organ-pipe has a stopper in the end of it, the air of condensation 
cannot get out, but is turned backwards, and there is no room for a 
new wave to begin until the first one has reached the mouth again ; 
in other words, each wave has to travel the length of the pipe twice 
if it is a ‘stopped pipe” I shall have occasion to show that by an 
instrument called the Siren we can prove that any sound has twice 
the number of vibrations of the sound an octave below ; hence we 
derive the fact that a stopped organ pipe sounds an octave below an 
open one of the same length. 
The larger number of pipes in an organ are of this flue kind, 
the principle being precisely that of an ordinary whistle, where the 
vibrations are Started by a jet of air being thrown against a sharp 
edge, across which it vibrates like a flexible tongue. 
