mull over vs speculate

mull over

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

speculate

verb
  • To make an inference based on inconclusive evidence; to surmise or conjecture. 

  • To make a risky trade in the hope of making a profit; to venture or gamble. 

  • To anticipate which branch of code will be chosen and execute it in advance. 

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