It concerns in this country, the interests of four millions of people, who, from the necessity of the case, can have no voice in determining their own condition.
The sentence refers to the overwhelming significance of slavery to the country. It's a big, domineering thing that's hard to get around or get rid of. He isn't using the word in its more common modern sense: "Slavery -- it's great!"
Colwell favored protective tariffs. In that sense he opposed "free trade." But he could appreciate that free trade could benefits some areas and industries (and hurt others). So it's not surprising that he might pay tribute to how well the planters had made use of free trade to enrich themselves. That didn't mean that he was in favor of the policy -- he saw that not everyone benefited from free trade -- or that he was disparaging market economies.
On page 22-23, I do find this:
The idea now pretty extensively entertained in the South, that New York is fattening on Southern trade and business is an utter delusion: New York is not piling up "Northern profits on Southern wealth." It is a misconception, which no unprejudiced man can entertain, if he will take the trouble to examine.
That is what Colwell set out to refute and what he largely does refute.
I might have given the impression that Colwell was more opposed to slavery than he was. Colwell wasn't an abolitionist but he did have serious disagreements with the slaveowner's great friend, Thomas Kettell, about the interpretation of statistics and the policy implications that should be drawn from the numbers.
Nineteenth century people don't pass our standards of political decency, but that doesn't mean that they didn't have bitter conflicts and arguments about many different matters. Colwell set out to refute Kettell's economic fallacies and certainly does some damage to Kettell's specious argument.
Here are a few more of Colwell’s quotes from his pamphlet that I “cherry picked” for you.
From pg 51:
“We regard African slavery as it now exists in the South, as justifiable on sound social, humane, and Christian considerations.”
From pg. 55:
It is quite certain that slavery in the South is not understood and appreciated in the North or in Europe as it should be...
“...an institution which so much concerns the interests and well being of human beings...deserves to be studied.
“...slavery deserves a social code of is own”
“...there are many large slaveholders in the South.....that if brought together for conference...might produce a constitution for slaves....after such a measure...abolitionism could no longer exist.”
I do not see or hear any rebuttal to Kettell’s economic study in any of that from his pamphlet.
I'm having trouble following the discussion, but this quote does help.
If I understand: Kettell proposed Northerners were, in effect, stealing Southern wealth and Colwell tried to rebut that, right?
Googling Kettell & Colwell produces limited data, though it appears both publications are still available, for about $20 each.
I'll probably skip them...
Bottom line is, I'm not inclined to believe that Southern slave-masters were in any way being "ripped off" by Northern shippers & business interests, certainly in no way comparable to the degree of rip-off of slaves themselves!
But I would possibly concede that highly effective anti-North propaganda made Southerners feeeeeeeeel "ripped-off".