enquiry vs inquiry

enquiry

noun
  • Search for truth, information or knowledge. 

  • A question. 

inquiry

noun
  • Search for truth, information, or knowledge; examination of facts or principles; research; investigation 

  • The act of inquiring; a seeking of information by asking questions; interrogation; a question or questioning. 

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