Form of Monochord. 51 
in all positions; it is sufficient if the wheel comes into contact 
with the lever for a short interval of time. The degree 
of contact may also be easily regulated. At the proper dis- 
tance from the siren, and opposite to it, a wooden stand is 
placed, carrying a pulley-wheel turning very easily, such as 
the wheel of an Atwood’s machine. The wheel is adjusted at 
such a height that the thread which passes over it may be 
horizontal when stretched by means of weights placed in a 
little scale-pan attached to it. If the friction between the 
wheel and the lever is too great in the horizontal position of 
the thread, the support carrying the pulley-wheel may be 
raised a little, so that the horizontal arm of the lever may be 
pulled up a little and the vertical arm thus removed somewhat 
from the wheel. If the lever does not touch the wheel at all, 
the support must be lowered somewhat. 
When the friction has been properly regulated, the siren 
may be put into action and its note gradually raised. At first 
the motion of the thread may be followed through its separate 
phases with the eye, but when the velocity of rotation of the siren 
becomes greater there is to be observed only an uncertain trem- 
bling of the thread, and the friction of the lever on the wheel 
produces a noise ; then this suddenly ceases, and the thread is 
seen to be in stationary oscillation of the fundamental form. 
The amplitude is considerable, and with threads of 1 metre long 
is often of three fingers’ breadth, if the dimensions of the thread 
have been well chosen. As soon as the thread has settled 
down to perform its fundamental oscillation with constant 
amplitude, it is found to be very easy to maintain the velocity 
of the siren constant. Hvery one who has used a siren to 
determine the pitch of a note must have remarked how diffi- 
cult it is to maintain the note of a definite pitch if there is no 
arrangement for regulating the pressure of air. I must 
acknowledge that I had not expected to see this difficulty 
so easily removed ; and it is certainly a peculiar action of the 
friction between the lever and the wheel, though one easily to 
be explained*, which causes that a change in the pressure of 
the air delivered by the bellows produces only a change in the 
intensity of the note given by the siren, and not at once any 
change in the velocity of rotation—if this coincides with the ~ 
velocity of vibration of the thread. ‘The tone of the siren and 
the vibration of the thread can equally easily be maintained 
constant, when the velocity of rotation of the siren is increased 
* The thread performing its own proper vibration is, as it were, in a 
condition of rigidity, out of which itis not easily brought (unless its mass 
is altogether too small), and thus acts, by means of the friction between 
the lever and the wheel, as a regulator to the siren. 
Ki 2 
