flotation vs substructure

flotation

noun
  • A state of floating, or being afloat. 

  • The ability (as of a tire or snowshoes) to stay on the surface of soft ground or snow. 

  • A process of separating minerals by agitating a mixture with water and detergents etc; selected substances being carried to the surface in air bubbles. 

  • The launching onto the market of a tranche of stocks or shares, usually a new issue. 

substructure

noun
  • Synonym of base (“the forces and relations of production that produce the necessities and amenities of life”) 

  • The earth or gravel that the railway tracks are embedded in. 

  • The supporting part of a structure (either physical or organizational; the foundation). 

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