hydroelectricity vs steam power

hydroelectricity

noun
  • electricity produced from the energy of moving water 

steam power

noun
  • Power derived from water heated into steam, usually converted to motive power by a reciprocating engine (a steam engine) or turbine (a steam turbine). 

  • Means of transportation, or heavy equipment, based on such power, especially steamboats, steamships, or steam trains. 

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