What is "pear gauge"

Pear \Pear\ (p[^a]r), n. [OE. pere, AS. peru, L. pirum: cf. F. poire. Cf. Perry.] (Bot.) The fleshy pome, or fruit, of a rosaceous tree ( Pyrus communis), cultivated in many varieties in temperate climates; also, the tree which bears this fruit. See Pear family, below. Pear blight.

1. (Bot.) A name of two distinct diseases of pear trees, both causing a destruction of the branches, viz., that caused by a minute insect ( Xyleborus pyri), and that caused by the freezing of the sap in winter.
   --A. J. Downing.

2. (Zo["o]l.) A very small beetle ( Xyleborus pyri) whose larv[ae] bore in the twigs of pear trees and cause them to wither.

   Pear family (Bot.), a suborder of rosaceous plants ( Pome[ae]), characterized by the calyx tube becoming fleshy in fruit, and, combined with the ovaries, forming a pome. It includes the apple, pear, quince, service berry, and hawthorn.

   Pear gauge (Physics), a kind of gauge for measuring the exhaustion of an air-pump receiver; -- so called because consisting in part of a pear-shaped glass vessel.

   Pear shell (Zo["o]l.), any marine gastropod shell of the genus Pyrula, native of tropical seas; -- so called from the shape.

   Pear slug (Zo["o]l.), the larva of a sawfly which is very injurious to the foliage of the pear tree.

n. A kind of gauge for measuring the exhaustion of an air-pump receiver.