(4) Relative pronoun, after a negative, stands for that ... not, or who ... not.
There is not a man in them but is impelled withal, at all moments, towards order.—Carlyle.
(5) Adverb, meaning only.
The whole twenty years had been to him but as one night.—Irving.
To lead but one measure.—Scott.
AS.
332. (1) Subordinate conjunction: (a) Of time.
Rip beheld a precise counterpart of himself as he went up the mountain.—Irving.
(b) Of manner.
As orphans yearn on to their mothers,
He yearned to our patriot bands.
—Mrs Browning.