A hydroscope is any of several instruments related to

water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...

: *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 which 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 technique. *An instrument (likely a hydrometer) described by Synesius in his ''Letter 15'' to

Hypatia Hypatia, Koine pronunciation (born 350–370; died 415 AD) was a neoplatonist philosopher, astronomer, and mathematician, who lived in Alexandria, Egypt, then part of the Eastern Roman Empire. She was a prominent thinker in Alexandria where ...

, 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 is then measured. Water clocks are one of the oldest time- ...

or ''clepsydra''.

Sources and notes

Optical instruments {{tech-stub