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 arcs 

 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. 



The arms. These are represented by Vb, Tb, 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 i 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 t, 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. XIII. PART II. 3 K 



