What does the word mainspring mean?

Usage examples for mainspring

  1. Now that she was going back, she was without this mainspring of hope- her old studies and pursuits wearied her. – Wife in Name Only by Charlotte M. Braeme (Bertha M. Clay)
  2. 3. A key- note of sincerity should be the mainspring of a well- built speech. – The Century Handbook of Writing by Garland Greever Easley S. Jones