The number of persons engaged in the slave-trade, and the amount of capital embarked in it, exceed our powers of calculation. The city of New York has been until of late the principal port of the world for this infamous commerce; although the cities of Portland and Boston are only second to her in that distinction. Slave dealers added largely to the wealth of our commercial metropolis; they contributed liberally to the treasuries of political organizations, and their bank accounts were largely depleted to carry elections in New Jersey, Pennsylvania, and Connecticut. It was natural for the leaders of the party which they had aided, to accord to them, as an equivalent, many facilities for carrying on their business. There is indeed no occasion for wonder at the countenance and impunity long given to such auxiliaries. If a few of them chanced to be of Knickerbocker stock, and to bear the talisman which affords admission to the higher circles of Gothamite respectability, it is only what might have been expected. There are such men everywhere, even in the Tombs. It requires no miraculous gift to be able to perceive why the late administration at Washington was sensitive as to the visitation of American vessels of doubtful character, by the officers of British cruisers. There was no principle at stake; but the slave-dealing interest had demanded as an immunity, that the piece of bunting known as the American flag should be allowed to protect from scrutiny every suspicious ship over which it should be raised. They had the power or influence to command; and the administration obeyed. The present administration appears to have awakened somewhat to this subject. The principal appointments for the Atlantic ports were given to men of anti-slavery proclivities. The new marshal of the southern district of New York was of different material from his predecessors, and fortunately he was no novice. He was familiar with the habits of the men engaged in the slave trade; he was ambitious and eager to signalize himself for efficiency. In three months he had seized nine vessels, and arrested twenty-eight men who had outfitted, commanded, or served on them. The Secretary of the Interior now resolved that the business should be broken up in every port of the United States. He accordingly issued an order to the several marshals of the States and districts lying upon the seaboard, directing them to assemble at the city of New York, on the fifteenth day of August, 1861, for the purpose of agreeing upon a system of measures for the effectual suppression of the slave-trade in American ports.