millpond vs piscina

millpond

noun
  • A pond or reservoir produced by damming a river or stream in order to provide a steady source of water for a millrace. 

piscina

noun
  • A basin or tank, especially one for holding fishes or for growing aquatic plants. 

  • A drained basin near a church's altar for the disposal of water from liturgical ablutions. 

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