Here he is suggesting that once Jefferson Davis completes removing the French that the South would be given a new set of states carved out of Mexico and Central America to the Isthmus (Panama) and blending of waters(a future canal?) to realign the unequal number of Northern vs Southern states and restore political balance. Holycow!

"If in delivering Mexico he should model its States in form & principle to adapt them to our Union and add a new Southern constellation to its benignant sky while rounding off our possession on the continent at the Isthmus, and opening the way to blending the waters of the Atlantic and Pacific, thus embracing our Republic in the arms of the Ocean, he would complete the work of Jefferson who first set one foot of our colossal government on the Pacific by a stride from the Gulf of Mexico. Such achievement would be more highly appreciated in the South, in as much as it would restore the equipoise between the Northern and Southern States; if indeed such sectional distinctions could be recognized, after the peculiar Institution which created them had ceased to exist."

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David Upton