A short scale made to slide along the divisions of a
   graduated instrument, as the limb of a sextant, or the scale of a
   barometer, for indicating parts of divisions. It is so graduated that a
   certain convenient number of its divisions are just equal to a certain
   number, either one less or one more, of the divisions of the
   instrument, so that parts of a division are determined by observing
   what line on the vernier coincides with a line on the instrument.