Where does the "rule of thirds" come from, and what did J. T. Smith actually write in 1797?

The phrase "Rule of thirds" appears, in quotation marks and with an apology for coining it, in J. T. Smith's 1797 Remarks on Rural Scenery. In the 1797 text it is a rule about proportions of light and shade and of areas (sky vs. earth/water, about 2:1), not a grid of four intersection points. Smith presents it as his own observation, derived by analogy from an observation about Rembrandt-type pictures, and invites correction; the OCR text read does not contain "golden". Curator qualifications: 4 of 4 excerpts present. Passage: 'Analogous to this " Rule of thirds ", ( if I may be allowed so to call it ) I have presumed to think that, in connecting or in breaking the various lines of a picture, it would likewise be a good rule to do it, in general, by a similar scheme of proportion ; for example, in a design of landscape, to determine the sky at about two-thirds ; or else at about one-third'. After CR-strip and hyphen-join: 'golden' 0, 'thirds' 6. Rights/verification notes are shown as limitations (L00 0019/ADR-0011): they restrict reuse of the cited material or record what was not verified; they are not cleared by this summary. Limitations: [component_reuse_restriction] OCR text of a Getty scan; IA metadata shows no licence field; public-domain status not concluded. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

{
  "citations": [
    {
      "url": "https://archive.org/download/remarksonruralsc00smit/remarksonruralsc00smit_djvu.txt",
      "note": "Internet Archive remarksonruralsc00smit_djvu.txt (equals producer pin), 'Of Light and Shade' pp. 15-17"
    }
  ]
}