Adams saying that he and 1774 Congress were super ignorant.
Between 1774 and 1797, an interval of twenty-three years, this ignorance was in some measure removed from some minds. But some had retired in disgust, some had gone into the army, some had been turned out for timidity, some had deserted to the enemy, and all the old, steadfast patriots, weary of the service, always irksome in Congress, had retired to their families and States, to be made governors, judges, marshals, collectors, &c., &c. So that in 1797, there was not an individual in the House of Representatives, in the Senate, or in either of the executive departments of government, who had been in the national controversy from the beginning.
Adams saying that the 1774 Congress guys got less ignorant over time, but there was big turnover, so by 1797 Congress and the Executive were all ignorant newb statesmen again. (Side note: this letter was literally written in the middle of the War of 1812 with the U.K. so that may be some relevant background here — like explaining how relations came to become bad with UK again).
Mr. Jefferson himself, the Vice-President, the oldest in service of them all, was but a young and a new man in comparison with the earliest conductors of the cause of the country, the real founders and legitimate fathers of the American republic.
Even Jefferson, who was like an elder statesman in 1797, was young newb compared to “real founders” of America.
The most of them had been but a very few years in public business, and a large proportion of these were of a party which had been opposed to the revolution, at least in the beginning of it.
So I think this is referring to the newb 1797 statesmen. Adams is saying they’re newbs, and a lot of them were opposed to the Revolution.
If he’s talking about the French Revolution, this seems like a big topic change.
If I were called to calculate the divisions among the people of America, as Mr. Burke did those of the people of England,
(Any Burke experts reading this, please provide insight here as to what this is referring to)
I should say that full one third were averse to the revolution.
Given the preceding context, (but in light of not knowing what the Burke reference is about), I have two thoughts:
1. This appears to be about the American Revolution, not the French Revolution. Adams had just been talking about 1797 Congressnewbs and how a bunch of them (or their party, at least), who had replaced the “real founders” of America, had opposed the Revolution.
2. I think the most natural reading is that he is talking about the people’s sentiment towards the Revolution at the time it happened, and not in 1797. Cuz he was just talking about the 1797 newb statesman’s (or their party’s) attitudes towards the revolution “at least at the beginning of it.”
These, retaining that overweening fondness, in which they had been educated, for the English, could not cordially like the French;
Note that part of the context of the American revolution was that, from a pretty early point, we were getting help from the French. So this could explain the issue of framing stuff in terms of liking England vs France. And in particular in terms of the anti-Revolution side being pro-English and the pro-Revolution side being pro-French.
indeed, they most heartily detested them. An opposite third conceived a hatred of the English, and gave themselves up to an enthusiastic gratitude
Gratitude sounds like something you give someone who helps you. Not something you give to somebody who is doing something you approve of. Of course, the connotations of the word could have changed over time.
to France. The middle third, composed principally of the yeomanry, the soundest part of the nation, and always averse to war, were rather lukewarm both to England and France;
See my comments above and also note Adams’ framing … on the one hand, you have some people detesting the French. On the other hand, you have some people hating the English. And then you have the calm yeoman farmer guys, averse to war and not really hating either side.
and sometimes stragglers  from them, and sometimes the whole body, united with the first or the last third, according to circumstances.
I have trouble interpreting this last bit. It could be about attitudes shifting during the war of independence but could be about stuff over time.
The depredations of France upon our commerce, and her insolence to our ambassadors,
The ambassadors thing could be reference to XYZ Affair, which happened in 1797. So what he’s talking about seems later in time now.
and even to the government, united, though for a short time, with infinite reluctance, the second third with the first,
So this I read as him saying that the factions he describes shift over time. Since he described the anti-revolution, pro-English faction first, and the pro-Revolution, anti-English faction second, he’s saying that France acted so badly that they managed to get these two sides together for a bit, politically.
and produced that burst of applause to the administration, in which you concurred, though it gave much offence to Mr. Randolph.
Overall I think Adams was talking about the attitudes to the American revolution, but the letter seemed significantly more ambiguous than I’d have expected given the treatment of the topic from both the “HISTORICAL FACT: Adams said opinion on American revolution was split 33-33-33” side, as well as the “HISTORICAL MYTH DEBUNKED” side.
But even IF Adams was talking about American attitudes to the American revolution, his like vague general impressions of people’s opinion is pretty poor evidence for making any kind of generalizations of what people thought of the revolution at the time.