What is "troughton level"

Dumpy level \Dump"y lev"el\ (Surv.) A level having a short telescope (hence its name) rigidly fixed to a table capable only of rotatory movement in a horizontal plane. The telescope is usually an inverting one. It is sometimes called the Troughton level, from the name of the inventor, and a variety improved by one Gavatt is known as the Gavatt level.