bivy vs laager

bivy

verb
  • To camp in a bivouac. 

noun
  • A bivouac sack. 

laager

verb
  • To camp in a circular formation. 

  • To arrange in a circular formation for defence. 

noun
  • A defensive encampment encircled by wagons, especially by South African Boers. 

  • A temporary formation of armoured vehicles for resupply. 

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