_From a letter of the 19th of April._--That Count de Vergennes had transmitted to the Minister the details respecting the proposed mediation of the Courts of Vienna and Petersburg. The Minister observed, that this had already been communicated to Congress through a committee. He repeated, for our recollection, that the acceptance on the part of France of the proposed mediation depended entirely, at that time, on the concurrence of the United States; and that with respect to Spain, its conduct would be determined by the dissolution of the negotiation with Mr Cumberland. That the Court of Spain had informed the Court of London, on the first proposal of the mediation of the Imperial Courts, that as a direct negotiation with the King of Great Britain was opened through Mr Cumberland, a mediation was unnecessary. That affairs since that time had taken a different turn. Mr Cumberland has been recalled; and the Catholic King, being now entirely at liberty, has accepted the mediation of the two Imperial Courts. That the King, our ally, had done the same; but that both Kings had declared at the same time to the two mediators, that the mediation could not possibly have any activity, without previously establishing some fundamental preliminaries. Of this observation the two mediating Courts had already felt the propriety. That France was then (viz. the 19th of April) expecting the effect, which this communication would produce on the Court of London. That the first question proposed by France, related to _the admission of an American Plenipotentiary_; and that the object of the second was to know, _upon what footing the King of England intended to treat with the United States_. The Court of France, not knowing that the United States had agreed to accept the mediation, again invite us to it. The Minister here observed, that Congress would judge by the questions proposed to the mediating powers, by the King his master, of the principles, by which his Majesty was actuated with respect to the United States. He assured us, that his Majesty is invariably resolved to abide by those principles, and will enter into no negotiation whatever before he shall receive a satisfactory solution of those two questions. He added, that the French Ministry trusted, that this conduct would more and more convince the United States, and would cause them to imitate the example of the King, and to feel that their honor and their interest call for their constant attachment, their friendship, and unreserved confidence towards him. The Count de Vergennes observes, that it is plain from these circumstances, that the negotiation for peace will be full of difficulties, and that it will probably be necessary, in pretensions as well as proceedings, to be very cautious, and to act so as to remove those difficulties, and everything, which might increase the acrimony, to which the English Plenipotentiaries may be naturally inclined.