I just discovered the Ngram Viewer, which displays the frequency of word use in the Google Books database. What an amazing tool for tracking social trends. It is particularly fascinating to enter companion terms, such as "men" and "women," "liberal" and "conservative," or "Darwin," "Freud," "Einstein," and "Marx." to see how the relative frequency of usage varies over time. Or, as a measure of literary/cultural reputation, see how mentions of Jane Austen suddenly and decisively overtake mentions of Charles Dickens beginning in the 1960s. As to the obsessions of this blog, take a look as the striking upsurge of the occurrence of the phrase "copyright infringement" in recent decades.
I could play with this for days.
(An earlier version of this post mentioned Thoreau overtaking Emerson in the 1960s in mentions in Google Books; that was a mistake; I failed to take into account that the searches are case sensitive.)