[Illustration: Fig. 157.]
This instrument is used to mark lines which are neither horizontal nor
vertical (usually these are drawn by the square and set-square), and
which are parallel to one another; by adjusting the edge of the parallel
ruler to a line, it can be extended or opened out (or vice-versa
closed), and the line or lines drawn will be parallel or equally distant
from the base or first line it was set by. See fig. 157.
Fig. 158 is a parallel ruler, constructed with two rollers fixed on a
rod so that they move the same distance, carrying the ruler parallel to
the starting line.
[Illustration: Fig. 158.]
NOTE.--It has been said that “a workman may be known by his tools,”
but the statement must be taken with a good deal of allowance. Some
workmen may possess a very fine set of tools and never use them,
because they have not the ability or inclination to learn how;
especially is this the case with drawing instruments.
If all the fine sets of drawing instruments that are owned by workmen
were put to frequent use the owners would find a marked improvement in
their abilities in other lines as well as drawing; for it is a
noticeable fact that when a person’s mind has been trained in a
business that requires close calculation and a knowledge of materials,
he is capable of showing good qualifications in other lines, and the
more skilled he is in one the easier can he acquire skillfulness in
another, if he applies the same amount of energy, thought and interest
as he did to acquire skill in the first.