water glass vs water clock

water glass and water clock both are nouns.

Word	Noun	Adjective	Verb	Adverb
water glass	Yes	No	No	No
water clock	Yes	No	No	No

As nouns, water glass and water clock are synonyms defined as:

water glass and water clock: clock that measures time by the escape of water

Other synonyms of water glass include clepsydra.

water glass (noun)	water clock (noun)
a glass for drinking water	clock that measures time by the escape of water
gauge for indicating the level of water in e.g. a tank or boiler or reservoir
clock that measures time by the escape of water
a viscous glass consisting of sodium silicate in solution; used as a cement or as a protective coating and to preserve eggs