ADJECTIVE (2)

1. occurring without interruption; chiefly restricted to what recurs regularly or frequently in a prolonged and closely spaced series;
- Example: "the continual banging of the shutters"

2. `continual' (meaning seemingly uninterrupted) is often used interchangeably with `continuous' (meaning without interruption);

The Collaborative International Dictionary of English v.0.48:

Continual \Con*tin"u*al\, a. [OE. continuel, F. continuel. See
   Continue.]
   1. Proceeding without interruption or cesstaion; continuous;
      unceasing; lasting; abiding.
      [1913 Webster]

            He that is of a merry heart hath a continual feast.
                                                  --Prov. xv.
                                                  15.
      [1913 Webster]

   2. Occuring in steady and rapid succession; very frequent;
      often repeated.
      [1913 Webster]

            The eye is deligh by a continental succession of
            small landscapes.                     --W. Irwing.
      [1913 Webster]

   Continual proportionals (Math.), quantities in continued
      proportion. --Brande & C.

   Syn: Constant; prepetual; incessant; unceasing;
        uninterrupted; unintermitted; continuous. See
        Constant, and Continuous.
        [1913 Webster]

WordNet (r) 3.0 (2006):

continual
    adj 1: occurring without interruption; chiefly restricted to
           what recurs regularly or frequently in a prolonged and
           closely spaced series; "the continual banging of the
           shutters" [ant: sporadic]
    2: `continual' (meaning seemingly uninterrupted) is often used
       interchangeably with `continuous' (meaning without
       interruption)

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "continual":
   ageless, articulated, catenated, ceaseless, chattering, coeternal,
   concatenated, connected, constant, continued, continuing,
   continuous, cyclical, dateless, direct, endless, eternal, eterne,
   ever-being, ever-durable, ever-during, everlasting, everliving,
   featureless, gapless, immediate, immemorial, incessant,
   indestructible, infinite, interminable, joined, jointless, linked,
   machine gun, monotonous, never-ceasing, never-ending, nonstop,
   nonterminating, nonterminous, olamic, oscillating, perdurable,
   perennial, periodic, permanent, perpetual, persistent, pulsating,
   rapid, recurrent, regular, repeated, repetitive, round-the-clock,
   running, seamless, sempiternal, serried, smooth, stable, staccato,
   steady, straight, stuttering, sustained, timeless,
   twenty-four-hour, unbroken, unceasing, unchanging,
   undifferentiated, unending, uniform, unintermitted, unintermittent,
   unintermitting, uninterrupted, unrelieved, unremitting, unstopped,
   unvarying, vibrating, without end