It keeps eternal whisperings around
Desolate shores, and with its mighty swell
Gluts twice ten thousand Caverns, till the spell
Of Hecate leaves them their old shadowy sound.
Often 'tis in such gentle temper found,
That scarcely will the very smallest shell
Be moved for days from where it sometime fell.
When last the winds of Heaven were unbound.
Oh, ye! who have your eyeballs vexed and tired,
Feast them upon the wideness of the Sea;
Oh ye! whose ears are dinned with uproar rude,
The calm present is contrasted with the ‘winds of Heaven’ which were ‘unbound’. This suggests a tension, the wind being reined in only to be released, or ‘unbound’ by a heavenly force to produce a storm which will shift shells and rocks and waves. 
 The full-stop after ‘fell’ forms a  caesura , a break where the reader pauses before moving on. This constitutes the  volta  or turn, after which the speaker begins a new approach.