] `There is a very clever essay in one of the books upstairs upon much such a subject, about young girls that have been spoilt for home by great acquaintance--The Mirror, I think'.... Anyone here know anything about the essay, or The Mirror?

I have four copies of The Mirror, which was a bound literary anthology containing short novels, biographies, memoirs, essays and suchlike. I don't have the issue containing that particular essay, alas. (Mind you, I do have the volume containing Clara Reeve's The Old English Baron - A Gothic Story. In her introduction, Reeve describes this new form of literature in some detail. I think it's the first use of the word Gothic in a literary sense. I'll copy out the intro tomorrow, if it isn't already available online.)

Anyway, the OUP edition of NA has the following footnote about The Mirror essay which Mrs Morland tries to find:

'The Mirror' - doubtless No. XII, Saturday, March 6 1779: Consequence to little folks of intimacy with great ones, in a letter form John Homespun. The habits of dissipation and almost of profligacy (cards on Sunday and a doubt of the immortality of the soul) contracted by Mr. Homespun's daughters during a visit to Lady_________ are sufficiently unlike poor Catherine's listlessness or her fancy for French-bread. We may suspect that Miss Austen is amusing herself at Mrs. Morland's expense, if not at ours.