retrocession vs sinking

retrocession

noun
  • The transfer of risk from one reinsurer to another. 

  • The act of retroceding; a going back. 

  • The return of land, rights, etc. previously ceded. 

  • Metastasis of an eruption or tumour from the surface to the interior of the body. 

sinking

noun
  • The process by which something sinks, or is sunk. 

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