let down vs touch bottom

let down

verb
  • To allow to descend. 

  • To disappoint; to betray or fail somebody 

  • To reduce one's level of effort. 

  • To soften in tempering. 

  • Used other than figuratively or idiomatically: see let, down. 

  • To lengthen by undoing and resewing a hem. 

touch bottom

verb
  • To reach the lowest point. 

How often have the words let down and touch bottom occurred in a corpus of books? (source: Google Ngram Viewer )