Sentence:  The machine represented in the frontispiece to this work, was invented for Mrs. Berkley to flog gentlemen upon, in the spring of 1828. ... There is a print in Mrs. Berkley's memoirs, representing a man upon it quite naked. A woman is sitting in a chair exactly under it, with her bosom, belly, and bush exposed: she is "manualizing" his <WSD>"embolon</WSD>" [plug], whilst Mrs. Berkley is birching his posteriors. 
Meaning of the word marked with <WSD> tags:  Anything wedge-shaped.