Executive at Washington appears to shrink from bringing forward, in any shape, a question, upon which depends the completion of their former object—­the utter and universal Abolition of the Slave Trade—­from an apprehension of alarming the Southern States."[49] Great Britain now offered to sign the proposed treaty of 1824 as amended; but even this Forsyth refused, and stated that the United States had determined not to become “a party of any Convention on the subject of the Slave Trade."[50]

Estimates as to the extent of the slave-trade agree that the traffic to North and South America in 1820 was considerable, certainly not much less than 40,000 slaves annually.  From that time to about 1825 it declined somewhat, but afterward increased enormously, so that by 1837 the American importation was estimated as high as 200,000 Negroes annually.  The total abolition of the African trade by American countries then brought the traffic down to perhaps 30,000 in 1842.  A large and rapid increase of illicit traffic followed; so that by 1847 the importation amounted to nearly 100,000 annually.  One province of Brazil is said to have received 173,000 in the years 1846-1849.  In the decade 1850-1860 this activity in slave-trading continued, and reached very large proportions.

The traffic thus carried on floated under the flags of France, Spain, and Portugal, until about 1830; from 1830 to 1840 it began gradually to assume the United States flag; by 1845, a large part of the trade was under the stars and stripes; by 1850 fully one-half the trade, and in the decade, 1850-1860 nearly all the traffic, found this flag its best protection.[51]

72. The Quintuple Treaty, 1839-1842. In 1839 Pope Gregory XVI. stigmatized the slave-trade “as utterly unworthy of the Christian name;” and at the same time, although proscribed by the laws of every civilized State, the trade was flourishing with pristine vigor.  Great advantage was given the traffic by the fact that the United States, for two decades after the abortive attempt of 1824, refused to co-operate with the rest of the civilized world, and allowed her flag to shelter and protect the slave-trade.  If a fully equipped slaver sailed from New York, Havana, Rio Janeiro, or Liverpool, she had only to hoist the stars and stripes in order to proceed unmolested on her piratical voyage; for there was seldom a United States cruiser to be met with, and there were, on the other hand, diplomats at Washington so jealous of the honor of the flag that they would prostitute it to crime rather than allow an English or a French cruiser in any way to interfere.  Without doubt, the contention of the United States as to England’s pretensions to a Right of Visit was technically correct.  Nevertheless, it was clear that if the slave-trade was to be suppressed, each nation must either zealously keep her flag from fraudulent use, or, as a labor-saving device, depute to others this duty for limited places