Lengthen, v. t. [imp. & p. p. Lengthenel; p. pr. & vb. n. Lengthening.]
To extent in length; to make longer in extent or duration; as, to lengthen a line or a road; to lengthen life; -- sometimes followed by out. What if I please to lengthen out his date. Dryden.
1 other Webster entry under this spelling
Lengthen, v. i.
To become longer. Locke.
Shades of meaning
Fernald has no entry of its own for lengthen; he lists it under protract.
He lists it without comment; the entries above explain its neighbors.