assistant vs second in command

assistant

noun
  • A software tool that provides assistance in some task, a wizard program. 

  • Sales assistant. 

  • A person who assists or helps someone else. 

adj
  • Helping; lending aid or support; auxiliary. 

  • Having a subordinate or auxiliary position. 

second in command

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