isochronous vs perpetual

isochronous

adj
  • Happening at the same time; simultaneous. 

  • Happening at regular intervals; isochronal. 

  • of or pertaining to data associated with time-sensitive applications. 

  • Of or pertaining to the use of clocks derived from the same clock reference. 

perpetual

adj
  • Continuing; uninterrupted. 

  • Set up to be in effect or have tenure for an unlimited duration. 

  • Lasting forever, or for an indefinitely long time. 

  • Flowering throughout the growing season. 

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