sandglass vs hourglass

sandglass and hourglass both are nouns.

Word	Noun	Adjective	Verb	Adverb
sandglass	Yes	No	No	No
hourglass	Yes	No	No	No

As nouns, hourglass is a hyponym of sandglass; that is, hourglass is a word with a more specific, narrower meaning than sandglass:

sandglass: timepiece in which the passage of time is indicated by the flow of sand from one transparent container to another through a narrow passage
hourglass: a sandglass that runs for sixty minutes

Other hyponyms of sandglass include egg timer.

sandglass (noun)	hourglass (noun)
timepiece in which the passage of time is indicated by the flow of sand from one transparent container to another through a narrow passage	a sandglass that runs for sixty minutes