and Description of the Eidograph. 437 
the other four. The rollers at the top and bottom of the tube T, 
turn on axes fixed in short projections, which form a part of it, as 
shewn in the figure. There is a spring, wu, passing along the side of 
the tube, which is fixed to it at the upper end, and loose at the lower. 
On this, the axis of the fifth roller turns when it is pressed upon 
by the pencil-case, which is thus acted on by three pressures in 
directions making equal angles round the axis. In this way the 
pencil is held steady while tracing on the paper, and little force 
is required to raise or depress it, because the rollers revolve on 
their centres by the pencil passing along their round surfaces, 
and the only material friction generated is that on their axes, 
which is but little in comparison of what it would be, if the pen- 
cil-case moved with equal freedom from shake in the tube T. 
There is a scaLe of equal parts on the upper surface of each 
arm, exactly like that on the beam, the divisions on all the three 
being of the same length, and numbered both ways from 0. 
Part of each scale is seen through an oblong opening in the 
wheels. There is an index engraved on a side of each opening, 
and the divisions are so numbered, that when the distances be- 
tween the axes of the tracer and copying pencil and the centres 
of motion of the wheels are each equal to one hundred divisions : 
then the zero divisions on the scales are opposite to the indices. 
There are vernier scales on the wheels exactly the same as that 
on the beam. The artist in constructing the instrument, takes 
care that the planes which pass along the axes of the tracer 
and copying points and the axes of motion of their wheels, be 
truly parallel ; when they are nearly so, he completes this adjust- 
ment by the swivel screws in the band. If the parallelism be 
disturbed by any accident, it may be restored in the same way. 
There is a mass of lead belonging to the instrument, which may 
be put on the shorter portion of the beam, as a saddle, when its 
centre is on one side of its point of support, as happens in mak- 
ing a considerably reduced copy. This acts as a counterpoise, 
and restores in some measure the equilibrium of the instrument 
on its base. 
3K 2 
