Illustrated Companion to the Latin Dictionary/Libratores

This article incorporates text from a publication now in the public domain: Rich, Anthony (1849). The illustrated companion to the Latin dictionary, and Greek lexicon. p. vi. OCLC 894670115. https://archive.org/details/illustratedcompa00rich. 

LIBRATO'RES. Professional persons, employed by the officers who had the superintendence of the public aqueducts, to make all the necessary surveys, ascertain the levels of different sources of water, and to regulate the size of the pipes which conveyed a supply of water from the reservoir (castellum) to the various establishments and houses of the city, in order that none might obtain more than their legal allowance; which was effected by calculating the quantity that would pass through a pipe of certain diameter in a given time. Plin. Ep. x. 70. 3. Frontin. Aq. 105.

2. In the army, soldiers who levelled and worked the machines from which missiles were discharged; like the engineers of modern warfare. Tac. Ann. ii. 20. xiii. 39.