The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

         What if I please to lengthen out his date. --Dryden.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Lengthen \Length"en\, v. i.
   To become longer. --Locke.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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