Having been disappointed, after considerable preparation, in the purpose of sending an exploring expedition up that river in the summer of 1804, it was thought best to employ the autumn in that year in procuring a knowledge on an interesting branch of the river called Washita. This was undertaken under the direction of Mr. Dunbar, of Natchez, a citizen of distinguished science, who had aided, and continues to aid us with his disinterested valuable services in the prosecution of these enterprises. He ascended the river to the remarkable hot springs near it, in latitude 34° 31´ 4´´.16, longitude, 92° 50´ 45´´ west, from Greenwich, taking its courses and distances, and correcting them by frequent celestial observations. Extracts from his observations, and copies of his map of the river, from its mouth to the hot springs, make part of the present communications. The examination of the Red river itself is but now commencing. SPECIAL MESSAGE.--MARCH 20, 1806. _To the Senate and House of Representatives of the United States_:-- It was reasonably expected, that while the limits between the territories of the United States and of Spain were unsettled, neither party would have innovated on the existing state of their respective positions. Some time since, however, we learned that the Spanish authorities were advancing into the disputed country to occupy new posts and make new settlements. Unwilling to take any measures which might preclude a peaceable accommodation of differences, the officers of the United States were ordered to confine themselves within the country on this side of the Sabine river; which, by the delivery of its principal post (Natchitoches), was understood to have been itself delivered up by Spain; and at the same time to permit no adverse post to be taken, nor armed men to remain within it. In consequence of these orders, the commanding officer of Natchitoches, learning that a party of Spanish troops had crossed the Sabine river and were posting themselves on this side the Adais, sent a detachment of his force to require them to withdraw to the other side of the Sabine, which they accordingly did. I have thought it proper to communicate to Congress the letters detailing this incident, that they may fully understand the state of things in that quarter, and be enabled to make such provision for its security as in their wisdom they shall deem sufficient. SPECIAL MESSAGE.--APRIL 14, 1806. _To the Senate and House of Representatives of the United States_:--