GEOTHERMOMETER
Etymology
Noun
geothermometer (plural geothermometers)
(geology) A proxy that allows for geothermometry, the determination of the temperature of a geological material from the past or existing at great depth.
(obsolete) A thermometer designed for measuring extremely high temperatures below ground level.
Source: Wiktionary
Ge`o*ther*mom"e*ter, n. Etym: [Gr. thermometer.] (Physics)
Definition: A thermometer specially constructed for measuring temperetures
at a depth below the surface of the ground.
Source: Webster’s Unabridged Dictionary 1913 Edition