A hydroscope is any of several instruments related to

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

: *One kind is an instrument for making observations below the surface of

, such as a long tube fitted with various

lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...

es arranged so that objects lying at the bottom can be reflected upon a screen on the deck of the ship that carries it. These are built with a large tire tube that supports the screen and covered by an acrylic dome for protection. *Another kind detects subsurface water through nuclear magnetic resonance using the

surface nuclear magnetic resonance Surface nuclear magnetic resonance (SNMR), also known as magnetic resonance Sounding (MRS), is a geophysical technique specially designed for hydrogeology. It is based on the principle of nuclear magnetic resonance (NMR) and measurements can be us ...

technique. *An instrument (likely a

hydrometer A hydrometer or lactometer is an instrument used for measuring density or relative density of liquids based on the concept of buoyancy. They are typically Calibration, calibrated and Graduation (instrument), graduated with one or more scales suc ...

) described by

Synesius Synesius of Cyrene (; ; c. 373 – c. 414) was a Greek bishop of Ptolemais in ancient Libya, a part of the Western Pentapolis of Cyrenaica after 410. He was born of wealthy parents at Balagrae (now Bayda, Libya) near Cyrene between 370 and 3 ...

in his ''Letter 15'' to

Hypatia Hypatia (born 350–370 – March 415 AD) was a Neoplatonist philosopher, astronomer, and mathematician who lived in Alexandria, Egypt (Roman province), Egypt: at that time a major city of the Eastern Roman Empire. In Alexandria, Hypatia was ...

, written in 402 AD. There are references to such instruments as early as the fourth century. *Another ancient Greek instrument: a

water clock A water clock, or clepsydra (; ; ), is a timepiece by which time is measured by the regulated flow of liquid into (inflow type) or out from (outflow type) a vessel, and where the amount of liquid can then be measured. Water clocks are some of ...

or ''clepsydra''.

Sources and notes

Optical instruments {{tech-stub