Sometimes the flooring of the wharves sagged and needed restoring to the former level.
Willis liked to see the pile-driver with its big hammer.

He marveled at the air-pumps with which sagging wharves were raised.

Perhaps three air-pumps at a time would be stationed over as many "caps," as the twelve-inch timbers under the wharf's flooring were called.

The pumps, being worked, would raise the caps and hold them until blocks could be shoved underneath.

Then the pumps were worked some more, and other blocks put under, till the wharf was restored to the required level.