and Description of the Eidograph. 433 



The base is a cylinder, externally of brass, but filled with 

 lead to give it stability ; two finger-screws pass through it, so 

 that, if thought necessary, it may be fixed to the table on which 

 the instrument stands when the instrument is used ; or the ends 

 of the screws may have each a sharp steel point, which may just 

 enter the wood, and keep the instrument from sliding on the 

 table. The shaded part of the section is a ring, to which the 

 short arms e e are fixed ; there are three of these, making equal 

 angles round the centre : only two, however, are seen, as in Fig. 

 1, the third being under the beam, and screwed to a strong plate 

 which connects the socket and beam. From the extremities of 

 these arms vertical rollers descend, Fig. 4. (m, m, are two of them), 

 which turn on their centres by the motion of the instrument, and 

 press on the upper surface of the base d d. Their use is to prevent 

 flexion of the main axis c, when the middle point of the beam is 

 on one side of its support, as happens in making a reduced or an 

 enlarged copy. There are adjustments a a, by which the weight 

 of the moveable part of the instrument is made to bear equally 

 on the three rollers, which thus transfer the weight from the axis 

 to the base. There is a screw in the lower end of the axis which 

 serves to give it greater or less tightness in the conical tube in 

 which it turns. Returning to the socket of the beam, / is a fin- 

 ger screw which passes diagonally through one of its angles. It 

 acts on a spring interposed between two sides of the beam (the 

 upper, and that opposite to the scale), and clamps it by pressing 

 it into the opposite angle formed by the other two. 



By drawing the beam along in the socket, its parts on each 

 side of the centre may have any assigned proportion to each 

 other ; and this is indicated by the scale on its side. Thus, when 

 the division 1 on the scale is placed opposite to the index on the 

 socket, one part of the beam has to the other the proportion of 

 99 to 101 ; and when 5 is opposite, the proportion is that of 95 

 to 105, or of 19 to 21, and so on. The rule to find the number 

 on the scale, which shall give a proposed proportion, is this : 



