either vs therewith

either

adv
  • As well. 

det
  • Any one (of two). 

  • Each of two; both. 

pron
  • One or the other of two people or things. 

conj
  • Introduces the first of two (or occasionally more) options or possibilities, the second (or last) of which is introduced by “or”. 

therewith

adv
  • In addition to that; besides, moreover. 

  • With this, that or those. 

  • Thereupon, forthwith; with that being said or done. 

How often have the words either and therewith occurred in a corpus of books? (source: Google Ngram Viewer )