The Collaborative International Dictionary of English v.0.48:
Ascertain \As`cer*tain"\, v. t. [imp. & p. p. Ascertained; p.
pr. & vb. n. Ascertaining.] [OF. acertener; a (L. ad) +
certain. See Certain.]
1. To render (a person) certain; to cause to feel certain; to
make confident; to assure; to apprise. [Obs.]
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When the blessed Virgin was so ascertained. --Jer.
Taylor.
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Muncer assured them that the design was approved of
by Heaven, and that the Almighty had in a dream
ascertained him of its effects. --Robertson.
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2. To make (a thing) certain to the mind; to free from
obscurity, doubt, or change; to make sure of; to fix; to
determine. [Archaic]
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The divine law . . . ascertaineth the truth.
--Hooker.
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The very deferring [of his execution] shall increase
and ascertain the condemnation. --Jer. Taylor.
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The ministry, in order to ascertain a majority . . .
persuaded the queen to create twelve new peers.
--Smollett.
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The mildness and precision of their laws ascertained
the rule and measure of taxation. --Gibbon.
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3. To find out or learn for a certainty, by trial,
examination, or experiment; to get to know; as, to
ascertain the weight of a commodity, or the purity of a
metal.
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He was there only for the purpose of ascertaining
whether a descent on England was practicable.
--Macaulay.
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Moby Thesaurus II by Grady Ward, 1.0:
77 Moby Thesaurus words for "ascertain":
afford proof of, appraise, ask, assure, be informed,
become acquainted with, bring home to, broaden the mind, catch on,
certify, cinch, clear up, clinch, consider, contemplate,
cram the mind, decide, demonstrate, determine, discover,
dismiss all doubt, ensure, establish, find, find out,
find out about, fix, follow, follow from, gain knowledge, get,
get at, get hold of, have a case, hear, hold good, hold water,
inquire, inspect, insure, interrogate, learn, learn about,
load the mind, make a decision, make certain, make good,
make no doubt, make no mistake, make out, make sure, make sure of,
nail down, observe, pick up information, prove, prove to be,
prove true, query, question, reassure, remove all doubt, resolve,
see, see that, see to it, set at rest, settle, settle the matter,
show, sort out, study, survey, tumble, unearth, view, weigh