Source : Webster's Revised Unabridged Dictionary (1913)

Lengthen \Length"en\ (-'n), v. t. [imp. & p. p. {Lengthened}
   (-'nd); p. pr. & vb. n. {Lengthening} (-'n*[i^]ng).]
   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.

Lengthen \Length"en\, v. i.
   To become longer. --Locke.

Source : WordNet®

lengthen
     v 1: make longer; "Lengthen this skirt, please" [ant: {shorten}]
     2: become long or longer; "In Spring, the days lengthen" [ant:
        {shorten}]