score:3
Other references to this quote apply the date "as late as 1847." This is very likely a reference to Howard's 1847 work Barometrographia: Twenty Years' Variation of the Barometer in the Climate of Britain, Exhibited in Autographic Curves, with the Attendant Winds and Weather, and Copious Notes Illustrative of the Subject. Note "Autographic Curves" in the title.
Possibly it can be found in the introduction, forward, preface, or early chapter of that book. The full text of the book itself does not appear to be on Gutenberg.org, Hathitrust, Google Books, or Archive.org; it may not have ever been digitized.
The National Library of Scotland has a copy, as do several other libraries around the world, but it's probably held in a reserve section.
The Royal Collection Trust has more information about the book.
Getty Images has an illustrated plate from the book for sale.