circus ring vs proscenium

circus ring

noun
  • A circular arena in a circus, where the acts are performed. 

proscenium

noun
  • The row of columns at the front the scene building, at first directly behind the circular orchestra but later upon a stage. 

  • The stage area between the curtain and the orchestra. 

  • The stage area immediately in front of the scene building. 

  • A proscenium arch. 

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