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 spi'ing, u, 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. 



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