Google Ngram on "Christmas tree"

[Joel S. Berson]

 From another list, on an apparent burst of usage in the mid-1700s,
and an antedating of the OED:

>The spike here [at 1766] is a misdated hit.
>
>===
>Decision by debate
>Douglas Ehninger, Wayne Brockriede - 1766 - 420 pages - Snippet
>view
>If Christmas tree lights are wired in series circuit, one bumed-
>out lamp puts out every lamp in the entire strand. In a parallel
>proof circuit, on the other hand, each proof is independently
>connected with the final claim. ..
>...

No comment.
Joel

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