Sentence:  Unerringly impelling this dead, impregnable, uninjurable wall, and this most buoyant thing within; there swims behind it all a mass of tremendous life, only to be adequately estimated as piled wood is—by the <WSD>cord</WSD> ... 
Meaning of the word marked with <WSD> tags:  A unit of measurement for firewood, equal to 128 cubic feet (4 &times; 4 &times; 8 feet), composed of logs and/or split logs four feet long and none over eight inches diameter. It is usually seen as a stack four feet high by eight feet long.