These instruments, while they appear alike, have a separate use: the
dividers are used to space off distances and dimensions; especially are
they necessary in reading drawings made to scale. _Compasses_ are used
for describing circles, curves, etc., _dividers_ are used for marking
out spaces.
Two forms of the dividers are shown in figs. 169 and 170; the simplest,
plainest form is shown in fig. 169; these are used for rough spacings;
fig. 170 represents a pair of dividers fitted with an adjustable screw
controlled by a steel spring in one leg; by this a very exact
measurement can be made. Fig. 170 is intended to exhibit what is called
a “hair-spring divider.”