In my post in Austen L and Janeites yesterday responding to Diane Reynolds's question about the meaning of the narration in Ch. 42 of P&P describing Lydia's letters as being "much too full of lines under the words to be made public" , I suggested that the "lines" referred to were not underlines actually physically written in ink beneath words in Lydia's letters, but rather that this was a metaphor,
and the "lines" were themselves WORDS written in ink on lines in Lydia's letters.
Therefore this was a slightly cryptic equivalent to what we today would refer to as "writing between the lines", i.e., implications and veiled allusions communicated discreetly by Lydia to her confidante, Kitty, which were not understood by other members of the family, but which could be READ between the lines. Which is, by the way, what was a frequent occurrence in real life, in JA's surviving letters to CEA.
Anyway, I reflected further on this, feeling there must be more in the text of the novel related to this, and so I did some further searching, and, sure enough, I found the "bread crumbs" five chapters later, in Chapter 47, which, I think, pretty strongly corroborate my TEXTUAL interpretation in Chapter 42. Here in Ch. 47 we have Lizzy questioning Jane to find out what was known at Longbourne about Lydia's shenanigans:
"And till Colonel Forster came himself, not one of you entertained a doubt, I suppose, of their being really married?"
"How was it possible that such an idea should enter our brains! I felt a little uneasy -- a little fearful of my sister's happiness with him in marriage, because I knew that his conduct had not been always quite right. My father and mother knew nothing of that; they only felt how imprudent a match it must be. KITTY THEN OWNED, with a very natural triumph on knowing more than the rest of us, that IN LYDIA'S LAST
LETTER she had prepared her for such a step. She had known, it seems, of THEIR BEING IN LOVE with each other, many weeks."
"But not before they went to Brighton?"
"No, I believe not." END OF QUOTED EXCERPT
So, knowing JA's penchant for making subliminal connections spanning many chapters in her novel texts, it seems clear to me that Lydia's being in love with Wickham was one of the main "lines under the words" hinted at in Ch. 42, which Kitty was not about to "make public", i.e., "decode" for the benefit of her other sisters and her parents.
Giveaway - Our Man on Earth by Blaise
1 hour ago