mull over vs perpend

mull over

verb
  • To think deeply about something; to ponder, deliberate or ruminate. 

perpend

noun
  • A brick or stone that has its longest dimension perpendicular to the face of a wall, especially one that extends through the wall's entire thickness. 

  • A vertical joint (usually mortar) between bricks or blocks in a horizontal course. 

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