litany vs rogation

litany

noun
  • A ritual liturgical prayer in which a series of prayers recited by a leader are alternated with responses from the congregation. 

  • A prolonged or tedious list. 

rogation

noun
  • A deeply serious and somber prayer or entreaty. 

  • The demand, by the consuls or tribunes, of a law to be passed by the people; a proposed law or decree. 

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