(1):
(v. t.) To make (a thing) certain to the mind; to free from obscurity, doubt, or change; to make sure of; to fix; to determine.
(2):
(v. t.) To render (a person) certain; to cause to feel certain; to make confident; to assure; to apprise.
(3):
(v. t.) 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.