Read it through once

[21] In Sperm-whalemen with any considerable quantity of oil on board, it is a regular semi-weekly duty to conduct a hose into the hold, and drench the casks with sea-water; which afterwards, at varying intervals, is removed by the ship’s pumps. Hereby the casks are sought to be kept damply tight; while by the changed character of the withdrawn water, the mariners readily detect any serious leakage in the precious cargo.