Sentence:  I must go down to the seas again, to the vagrant gypsy life,/ To the gull's way and the whale's way, where the wind's like a whetted knife;/ And all I ask is a merry yarn from a laughing fellow-rover,/ And quiet sleep and a sweet dream when the long <WSD>trick</WSD>'s over. 
Meaning of the word marked with <WSD> tags:  A sailor's spell of work at the helm, usually two hours long.