at intervals
 不時地,時時,間隔一定的距離
 In·ter·val n.
 1. A space between things; a void space intervening between any two objects; as, an interval between two houses or hills.
 'Twixt host and host but narrow space was left,
 A dreadful interval.   --Milton.
 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.
 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.
 4. Mus. Difference in pitch between any two tones.
 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.
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