46 JOURNAL AND PROCEEDINGS. 
a centre ; in the circle described by the spot where the pipe meets 
it are perforated a certain number of holes ; and the mechanism for 
twirling the disc is such that you can tell exactly how many revolu- 
tions it makes, however fast it may be going. When the current of 
air is turned on and ‘the disc revolves, at first nothing is heard but 
detached puffs as each hole passes the end of the pipe and allows 
some air to go through; the puffs get more rapid until the untrained 
ear can count them no longer, and soon they develop into a low 
hum of a decided note ; the quicker goes the wheel the higher goes 
the note until a high velocity produces a perfect scream. By this it 
is proved that musical tone is nothing more than a succession of 
regular vibrations above a certain speed ; that the lowest number 
perceptible as tone by the average ear is 16 per second ; also that 
double that number makes the sound an octave higher. In a 
modern piano the lowest A has 274 vibrations per second ; each 
octave doubles that until the top A has 3520. How much music 
there is in these very high notes is an open question. These num- 
bers, however, are proved with mathematical exactitude by this 
ingenious contrivance. 
Having briefly considered the various means of starting musical 
vibrations, we come to the consideration of their transmission 
through various media, taking air as the first. Recognize, to begin 
with, this definition with regard to musical air waves: The pitch of 
sound (é.¢, its acuteness or gravity of tone) is dependent on the 
length of the waves in the direction of their travel ; the loudness of 
the sound is dependent on the amptiiude or width transversely to 
the line of travel. The instrument which initiates the tone estab- 
lishes the pitch (which remains unaltered) and, to a certain extent, 
the loudness too, though that will be materially altered by the 
medium through which it has to pass and the distance it has to 
travel. The pitch will sometimes be slightly altered, as a piano will 
sometimes sound out of tune when heard through a wall. 
Sound travels through air of ordinary temperature at about the 
rate of 1140 feet per second—38o yards. The colder the air the 
slower the travel, warmer air faster travel ; a difference, roughly, of 
one foot of speed for each degree of heat or cold. Light travels at 
the rate of 200,000 miles per second. So, as this means that all 
terrestrial distances are covered by light practically instantaneously, 
