“To adjust, or ascertain if the eidograph is in adjustment, is very simple, from the reason that when the arms are parallel the adjustment is perfect for all proportions. The manner of ascertaining this is as follows: place all the verniers at zero, which will bring them to the exact centres of the arms and the beam, place the arms at about right angles with the beam, then mark simultaneously with the tracer and pencil point, turn the instrument round upon its fulcrum, so that the pencil point be brought into the mark made by the tracer; then, if the tracer fall into the mark made by the pencil the instrument is in adjustment. If it should not fall into the same mark, the difference should be bisected, and the adjusting screws on the bands should be moved until the tracer fall exactly into the bisection, which will be perfect adjustment.
“When the eidograph is in adjustment, if the three verniers be set to the same reading on any part of their scale, the pencil point, fulcrum, and tracer will be in a true line. If it should not be so, it will show the dividing of the instrument to be inaccurate. Thus we have a simple way of testing the eidograph in every important particular.
“The divisions upon the eidograph do not positively indicate the reductions required to be performed by the instrument, but merely give a scale, which, with the assistance of the vernier, divides the beam and arms into 1000 parts. To obtain the quantity to which the verniers are to be set, it is necessary either to apply to a table of proportions relative to divisions, or to simple arithmetic, as will be shown. A printed table is very generally placed inside the lid of the box in which the instrument is packed, which contains part of the following proportions:
Table for Reducing or Enlarging Proportions.
| Proportions. | Divisions on Bars. | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| As | 1 | is to | 2 | 33 | ·333 | ||||
| „ | 1 | „ | 3 | 50 | |||||
| „ | 1 | „ | 4 | 60 | |||||
| „ | 1 | „ | 5 | 66 | ·666 | ||||
| „ | 1 | „ | 6 | 71 | ·428 | ||||
| „ | 1 | „ | 7 | 75 | |||||
| „ | 1 | „ | 8 | 77 | ·777 | ||||
| „ | 1 | „ | 9 | 80 | |||||
| „ | 1 | „ | 10 | 81 | ·818 | ||||
| „ | 2 | „ | 3 | 20 | |||||
| „ | 2 | „ | 5 | 42 | ·857 | ||||
| „ | 3 | „ | 4 | 14 | ·285 | ||||
| „ | 3 | „ | 5 | 25 | |||||
| „ | 4 | „ | 5 | 11 | ·111 | ||||
| „ | 5 | „ | 6 | 9 | ·09 | ||||
“The table here given answers for the general purposes of reducing, such as the bringing of a plan from one chain scale to another, the quantities of which are found by the following rule:
“To find the quantity equal to any given proportion for the setting of the eidograph.
—Subtract one sum of the proportion from the other, and multiply this difference by 100 for a dividend; add the two sums of the proportion together for a divisor: the quotient from the working of this will give the number to which the arms and beam are to be set.
“For instance, let it be required to reduce a drawing in the proportion of 3 to 5.
| 5 - 3 | = | 2 | ||
| × | 100 | |||
| 5 + 3 = 8 | ) | 200 | ( | 25 |