let down vs lower

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. 

lower

verb
  • To let descend by its own weight, as something suspended; to let down 

  • To decrease in value, amount, etc. 

  • To reduce operations to single machine instructions, as part of compilation of a program. 

  • To reduce the degree, intensity, strength, etc., of 

  • To bring down; to humble 

  • (lower oneself) To humble oneself; to do something one considers to be beneath one's dignity. 

  • To reduce the height of 

  • To depress as to direction 

  • to pull down 

  • To reduce (something) in value, amount, etc. 

  • To make less elevated 

  • To fall; to sink; to grow less; to diminish; to decrease 

adj
  • bottom; more towards the bottom than the middle of an object 

  • Situated on lower ground, nearer a coast, or more southerly. 

  • older 

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