knock on wood vs lay down the law

knock on wood

verb

To take a customary action to ward off some misfortune that is believed to be attracted by a presumptuous statement.

intj

Hopefully; a self-directive to undertake the customary action to ward off bad luck.

lay down the law

verb

To authoritatively or dogmatically assert what is permitted or not permitted.
To promulgate law.

How often have the words knock on wood and lay down the law occurred in a corpus of books? (source: Google Ngram Viewer )