at intervals

Interval \In"ter*val\, n. [L. intervallum; inter between +
   vallum a wall: cf. F. intervalle. See {Wall}.]
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   1. A space between things; a void space intervening between
      any two objects; as, an interval between two houses or
      hills.
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            'Twixt host and host but narrow space was left,
            A dreadful interval.                  --Milton.
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   2. Space of time between any two points or events; as, the
      interval between the death of Charles I. of England, and
      the accession of Charles II.
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   3. A brief space of time between the recurrence of similar
      conditions or states; as, the interval between paroxysms
      of pain; intervals of sanity or delirium.
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   4. (Mus.) Difference in pitch between any two tones.
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   {At intervals}, coming or happening with intervals between;
      now and then. "And Miriam watch'd and dozed at intervals."
      --Tennyson.

   {Augmented interval} (Mus.), an interval increased by half a
      step or half a tone. Interval