tremulous vs vacillating

tremulous

adj

  • Timid, hesitant; lacking confidence. 

  • Trembling, quivering, or shaking. 

vacillating

adj

  • Liable to vacillate. 

noun

  • Vacillation. 

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