By a letter from Joseph Chiappe, our agent at Mogadore, I am notified of a declaration of the Emperor of Morocco, that if the States General of the United Netherlands do not, before the month of May, send him an ambassador, to let him know whether it is war or peace between them, he will send one to them with five frigates; and that if their dispositions be unfavorable, their frigates shall proceed to America to make prizes on the Dutch, and to sell them there.  It seems to depend on the Dutch, therefore, whether the Barbary powers shall learn the way to our coasts, and whether we shall have to decide the question of the legality of selling in our ports vessels taken from them.  I informed you, in a former letter, of the declaration made by the court of Spain to that of London, relative to its naval armament, and also of the declaration of the Count de Montmorin to the Russian minister here on the same subject.  I have good information, that the court of Spain has itself made a similar and formal declaration to the minister of Russia at Madrid.  So that Russia is satisfied she is not the object.  I doubt whether the English are equally satisfied as to themselves.  The season has hitherto prevented any remarkable operation between the Turks and the two empires.  The war, however, will probably go on, and the season now admits of more important events.  The Empress has engaged Commodore Paul Jones in her service.  He is to have the rank of rear-admiral, with a separate command, and it is understood that he is in no case to be commanded.  He will probably be opposed to the Captain Pacha on the Black Sea.  He received this invitation at Copenhagen, and as the season for commencing the campaign, was too near to admit time for him to ask and await the permission of Congress, he accepted the offer, only stipulating, that he should be always free to return to the orders of Congress whenever called for, and that he should not be expected to bear arms against France.  He conceived, that the experience he should gain would enable him to be more useful to the United States, should they ever have occasion for him.  It has been understood, that Congress had had it in contemplation to give him the grade of rear-admiral, from the date of the action of the Serapis, and it is supposed, that such a mark of their approbation would have a favorable influence on his fortune in the north.  Copies of the letters which passed between him and the Danish minister are herewith transmitted.  I shall immediately represent to Count Bernstorff, that the demand for our prizes can have no connection with a treaty of commerce; that there is no reason why the claims of our seamen should await so distant and uncertain an event; and press the settlement of this claim.