I am publishing here Mark Davies’ announcement of COHA:
We are pleased to announce the release of the 400 million word Corpus of Historical American English (1810-2009). The corpus has been funded by a generous grant from the US National Endowment for the Humanities, and it is freely available at http://corpus.byu.edu/coha/. COHA is the largest structured corpus of historical English, and it contains more than 100,000 texts from fiction, popular magazines, newspapers, and non-fiction books, with the same genre balance decade by decade from the 1810s-2000s.
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