I've noticed that Ngrams feature quite often in threads I've been involved with (I've used it myself) but, allowing that the Ngram program can be useful, how cautious should we be in using it?
An extreme example is the phrase "fit down". This phrase was enormously popular in 1800, followed by a precipitous decline through 1820, virtually vanishing as we move from 1820-1840.
The "fit down" Ngram
Unfortunately, the tale of "fit down" is entirely the result of the google OCR program insisting in transcribing the "long-s" as "f". What we are seeing is the ghost of "sit down".
How wary should we be of the Ngram program? In just what ways is it useful? Should there be lodged, somewhere, a "Guide to Using Ngram"?