stepping-stone vs vehicle

stepping-stone

vehicle

noun
  • A liquid content (e.g. oil) which acts as a binding and drying agent in paint. 

  • An animal or (rarely) a plant on which a Hindu deity rides or sits 

  • An entity to achieve an end. 

  • A mode or method of spiritual practice; a yana. 

  • A medium for expression of talent or views. 

  • The main excipient (such as an oil or gel) that conveys the active ingredient of a drug. 

  • A conveyance; a device for carrying or transporting substances, objects or individuals. 

How often have the words stepping-stone and vehicle occurred in a corpus of books? (source: Google Ngram Viewer )