Meantime, it had become clear to many in the United States that the individual efforts of States could never suppress or even limit the trade without systematic co-operation.  In 1817 a committee of the House had urged the opening of negotiations looking toward such international co-operation,[29] and a Senate motion to the same effect had caused long debate.[30] In 1820 and 1821 two House committee reports, one of which recommended the granting of a Right of Search, were adopted by the House, but failed in the Senate.[31] Adams, notwithstanding this, saw constitutional objections to the plan proposed by Canning, and wrote to him, December 30:  “A Compact, giving the power to the Naval Officers of one Nation to search the Merchant Vessels of another for Offenders and offences against the Laws of the latter, backed by a further power to seize and carry into a Foreign Port, and there subject to the decision of a Tribunal composed of at least one half Foreigners, irresponsible to the Supreme Corrective tribunal of this Union, and not amendable to the controul of impeachment for official misdemeanors, was an investment of power, over the persons, property and reputation of the Citizens of this Country, not only unwarranted by any delegation of Sovereign Power to the National Government, but so adverse to the elementary principles and indispensable securities of individual rights, ... that not even the most unqualified approbation of the ends ... could justify the transgression.”  He then suggested co-operation of the fleets on the coast of Africa, a proposal which was promptly accepted.[32]

The slave-trade was again a subject of international consideration at the Congress of Verona in 1822.  Austria, France, Great Britain, Russia, and Prussia were represented.  The English delegates declared that, although only Portugal and Brazil allowed the trade, yet the traffic was at that moment carried on to a greater extent than ever before.  They said that in seven months of the year 1821 no less than 21,000 slaves were abducted, and three hundred and fifty-two vessels entered African ports north of the equator.  “It is obvious,” said they, “that this crime is committed in contravention of the Laws of every Country of Europe, and of America, excepting only of one, and that it requires something more than the ordinary operation of Law to prevent it.”  England therefore recommended:—­

1.  That each country denounce the trade as piracy, with a view of founding upon the aggregate of such separate declarations a general law to be incorporated in the Law of Nations.

2.  A withdrawing of the flags of the Powers from persons not natives of these States, who engage in the traffic under the flags of these States.

3.  A refusal to admit to their domains the produce of the colonies of States allowing the trade, a measure which would apply to Portugal and Brazil alone.

These proposals were not accepted.  Austria would agree to the first two only; France refused to denounce the trade as piracy; and Prussia was non-committal.  The utmost that could be gained was another denunciation of the trade couched in general terms.[33]