SIR,--Your favor of November 23d, after a very long passage, is received, and with it the map which you have been so kind as to send me, for which I return you many thanks. It is handsomely executed, and on a well-chosen scale; giving a luminous view of the comparative possessions of different powers in our America. It is on account of the value I set on it, that I will make some suggestions. By the charter of Louis XIV. all the country comprehending the waters which flow into the Mississippi, was made a part of Louisiana. Consequently its northern boundary was the summit of the highlands in which its northern waters rise. But by the Xth Art. of the Treaty of Utrecht, France and England agreed to appoint commissioners to settle the boundary between their possessions in that quarter, and those commissioners settled it at the 49th degree of latitude. See Hutchinson's Topographical Description of Louisiana, p. 7. This it was which induced the British Commissioners, in settling the boundary with us, to follow the northern water line to the Lake of the Woods, at the latitude of 49°, and then go off on that parallel. This, then, is the true northern boundary of Louisiana. The western boundary of Louisiana is, rightfully, the Rio Bravo, (its main stream,) from its mouth to its source, and thence along the highlands and mountains dividing the waters of the Mississippi from those of the Pacific. The usurpations of Spain on the east side of that river, have induced geographers to suppose the Puerco or Salado to be the boundary. The line along the highlands stands on the charter of Louis XIV. that of the Rio Bravo, on the circumstance that, when La Salle took possession of the Bay of St. Bernard, Panuco was the nearest possession of Spain, and the Rio Bravo the natural half-way boundary between them. On the waters of the Pacific, we can found no claim in right of Louisiana. If we claim that country at all, it must be on Astor's settlement near the mouth of the Columbia, and the principle of the _jus gentium_ of America, that when a civilized nation takes possession of the mouth of a river in a new country, that possession is considered as including all its waters. The line of latitude of the southern source of the multnomat might be claimed as appurtenant to Astoria. For its northern boundary, I believe an understanding has been come to between our government and Russia, which might be known from some of its members. I do not know it. Although the irksomeness of writing, which you may perceive from the present letter, and its labor, oblige me now to withdraw from letter writing, yet the wish that your map should set to rights the ideas of our own countrymen, as well as foreign nations, as to our correct boundaries, has induced me to make these suggestions, that you may bestow on them whatever inquiry they may merit. I salute you with esteem and respect. TO MRS. ADAMS. MONTICELLO, January 11, 1817.