locum vs second string

locum

noun
  • A share in a Genoese trading vessel. 

  • A period working as a locum tenens. 

second string

adj
  • Not as good; of a lower quality or condition. 

noun
  • A unit of players which is considered to have a secondary level of talent and which usually plays after the first string. 

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