Two negatives destroy one another, and are generally equivalent to an
affirmative; as, "Such things are _not un_common;" i.e. they are common.
NOTE. When one of the two negatives employed is joined to another
word, it forms a pleasing and delicate variety of expression; as,
"His language, though inelegant, is _not un_grammatical;" that is,
it is grammatical.
But, as two negatives, by destroying each other, are equivalent to
an affirmative, they should not be used when we wish to convey a
_negative_ meaning. The following sentence is therefore inaccurate:
"I can_not_ by _no_ means allow him what his argument must prove."
It should be, "I cannot by _any_ means," &c., or, "I _can_ by _no_
means."