and Description of the Eidograph. 433 
TuE 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 ams e e are fixed ; there are three of these, making equal 
angies 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 dd. 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 thisis 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: 
