…exhausting himself in attempts to get back to the mouth of the cove, in his weakness swimming several inches deeper than was his wont, keeping up his breathing entirely by his nostrils, turning upon his back a dozen times over, swimming en papillon, and so on, Troy resolved as a last resource to tread water at a slight incline, and so endeavour to reach the shore at any point, merely giving himself a gentle impetus inwards whilst carried on in the general direction of the tide.
There are no more uses of "impetus" in the book.
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Budget cuts were the impetus for the reorganization.
The Supreme Court decision provided the impetus for the change.