and Description of the Eidograph. 435 
THE BAND is composed partly of very thin and narrow watch- 
springs fh, fh, and partly of steel wires gg ; of course, the for- 
mer only can be applied to the circumferences, but neither wheel 
requires to be turned more than about half a revolution ; and 
there are stop-pins fixed vertically in them, which come against 
the beam and prevent them from turning farther. Hence the 
spring parts of the bands need not be much longer than the ares 
on which they are applied, and each is attached by soldering to 
a piece of brass screwed to the wheel, so that it cannot slide 
along the convex surface. The stops prevent the soldered points 
from ever being detached from the wheel. The bottoms of the 
grooves to which the band is applied ought to be truly cylindric 
surfaces, of exactly the same diameter, and concentric with their 
axes. The springs are connected by steel wires, the junctions are 
made by swivel screws k, k, by which the band may be tighten- 
ed, or one wheel turned round a little, while the other is at rest ; 
this last motion is required in the adjustment of the instrument. 
Tue arms, These are represented by Pd, Td, Fig. 1; they 
are hollow four-sided prisms of brass; their upper and lower 
faces are half an inch broad, and their other sides a quarter of an 
inch ; their length is about twenty-eight inches ; they fit into 
sOcKETS which are directly under the centre of the wheels, and 
go quite across them, so that the direction of the sliding motion 
of an arm in its socket is in a vertical plane, which should pass 
always along the same diameter of the wheel. When the instru- 
ment is used, each arm is firmly fixed to its wheel by a clamping 
screw. In Fig. 5, b is the socket of an arm, and 7 the head of its 
clamping screw. The sockets are slit and sprung, to allow the 
arms to enter and pass freely along them. The tracer T, the 
point of which is of steel, is fixed at the extremity of one arm, 
and the copying pencil P (the point is of black lead) at the extre- 
mity of the other arm. Figs. 2. and 3. represent the tracer and 
copying pencil of half their actual size. The tracer pp, Fig. 2, 
passes through a tube #, in which it slides, and there is a finger 
screw in the side of the tube to fix it at the proper height in 
VOL, XIIl. PART II. 3K 
