THE LOGOGRAPH, OR WRITING BY THE YOICE. 279 
A spring which carries the marker is made to press against 
the membrane with a slight initial pressure, to prevent as far 
as practicable the effects of jar and consequent vibratory 
action. 
A very light arm of aluminium is connected with the spring, 
and holds the marker ; and a continuous strip of paper is made 
to pass under the marker in the same manner as that employed 
in telegraphy. 
The marker consists of a small fine sable brush, placed in a 
light tube of glass of an inch in diameter. The tube is 
rounded at the lower end, and pierced with a hole about of 
an inch in diameter. Through this hole the tip of the brush is 
made to project, and it is fed by colour put into the glass tube 
in which it is held. To provide for the escape of the air pass- 
ing through the instrument, a small orifice is made in the side 
of the tube of the speaking-trumpet, so that the pressure 
exerted upon the membrane and its spring is that due to the 
difference arising from the quantity of air forced into the 
trumpet and that which can be delivered through the orifice in 
a given time. 
There being an initial pressure upon the membrane, to 
prevent vibratory action as I before described, the strength 
of the spring and the size of the orifice had to be adjusted, 
so that while the lightest pressures arising under articula- 
tion could be recorded, the greatest pressures should not pro- 
duce a movement exceeding the limit of the width of the 
paper. 
It will be seen that in this construction of the instrument 
the sudden application of pressure is as suddenly recorded, 
subject only to the modifications occasioned by the inertia, 
momentum, and friction of the parts moved. But the record 
of the sudden cessation of pressure is further affected by the 
time required to discharge the air through the escape orifice. 
Inasmuch, however, as these several effects are similar under 
similar circumstances, the same diagram should always be 
obtained from the same pneumatic action when the instrument 
is in proper adjustment ; and this result is fairly borne out by 
the experiments. 
We are thus enabled to trace to what extent the pneumatic 
action varies with different articulations ; and it will be seen 
that, although there are instances in which considerable differ- 
ences in sound do not make much variation in the diagram, yet, 
as a rule, every change of sound or articulation produces a 
