hydrant vs vacuum tube

hydrant

noun
  • An outlet from a liquid/fluid main often consisting of an upright pipe with a valve attached from which fluid (e.g. water or fuel) can be tapped. 

vacuum tube

noun
  • A type of electron tube that controls electron current through vacuum between electrodes within a sealed container, usually of glass or metal and often designed to be easily installed in and removed from the equipment in which it is used. 

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