Sentence:  For indeed, looking at the activity of the historic Pen and Press through this last half-century, and what bulk of History it yields for that period alone, and how it is henceforth like to increase in decimal or <WSD>vigesimal</WSD> geometric progression,—one might feel as if a day were not distant, when perceiving that the whole Earth would not now contain those writings of what was done in the Earth, the human memory must needs sink confounded, and cease remembering!— ... 
Meaning of the word marked with <WSD> tags:  Occurring in intervals of twenty. (from mid-17th c.)