MOUTH OF YELLOWSTONE, UPPER MISSOURI, 1838.

I arrived at this place yesterday on the steamer "Yellow Stone," after a voyage of nearly three months from St. Louis, a distance of two thousand miles, the greater part of which has never before been navigated by steam ; and the almost insurmountable difficulties which continually oppose the voyageur on this turbid stream, have been by degrees overcome by the indefatigable zeal of Mr. Chouteau, a gentleman of great perseverance, and part proprietor of the boat. To the politeness of this gentleman I am indebted for my passage from St. Louis to this place, and I had also the pleasure of his company, with that of Major Sanford, the government agent for the Missouri Indians.

The American Fur Company have erected here, for their protection against the savages, a very substantial Fort, 300 feet square, with bastions armed with ordnance; and our approach to it under the continued roar of cannon for half an hour, and the shrill yells of the half-affrighted savages who lined the shores, presented a scene of the most thrilling and picturesque appearance. A voyage so full of incident, and furnishing so many novel scenes of the picturesque and romantic, as we have passed the numerous villages of the "astonished natives", saluting them with the puffing of steam and the thunder of artillery, would afford subject for many epistles; and I cannot deny myself the pleasure of occasionally giving you some little sketches of scenes that I have witnessed, and am witnessing; and of the singular feelings that are excited in the breast of the stranger traveling through this interesting country. Interesting (as I have said) and luxurious, for this is truly the land of Epicures; we are invited by the savages to feasts of dog's meat, as the most honorable food that can be presented to a stranger, and glutted with the more delicious food of beaver tails, and buffaloe tongues. You will, no doubt, be somewhat surprised on the receipt of a Letter from me, so far strayed into the Western World; and still more startled, when I tell you that I am here in the full enthusiasm and practice of my art. That enthusiasm alone has brought me into this remote region, 3500 miles from my native soil; the last 2000 of which have furnished me with almost unlimited models, both in landscape and the human figure, exactly suited to my feelings. I am now in the full possession and enjoyments of Ghose conditions, on which alone I was induced to Pursue the art as a Profession; and in anticipation of which alone, my·admiration for the art could ever have been kindled into a pure flame. I mean the free use of nature's undisguised models, with the privilege of selecting for myself. If I am here losing the benefit of the fleeting fashions of the day, and neglecting that elegant polish, which the world say an artist should draw from a continual intercourse with the polite world; yet have I this consolation, that in this country, I am entirely divested of those dangerous steps and allurements which beset an artist in fashionable life; and have little to steal my thoughts away from the contemplation of the beautiful models that are about me. If, also, I have not here the benefit of that feeling of emulation, which is the life and spur to the arts, where artists are associates together; yet am I surrounded by living models of such elegance and beauty, that I feel an unceasing excitement of a much higher order -- the certainty that I am drawing knowledge from the true source. My enthusiastic admiration of man in the honest and elegant simplicity of nature, has always fed the warmest feelings of my bosom, and shut half the avenues to my heart against the specious refinements of the accomplished world. This feeling, together with the desire to study my art, independently of the embarrassments which the ridiculous fashions of civilized society have thrown in its way, has led me to the wilderness for a while, as the true school of the arts.

I have for a long time been of opinion, that the wilderness of our country afforded models equal to those from which the Grecian sculptors transferred to the marble such inimitable grace and beauty; and I am now more confirmed in this opinion, since I have immersed myself in the midst of thousands and tens of thousands of these knights of the forest; whose whole lives are lives of chivalry, and whose daily feats, with their naked limbs, might vie with those of the Grecian youths in the beautiful rivalry of the Olympian games.

No man's imagination, with all the aids of description that can be given to it, can ever picture the beauty and wildness of scenes that may be daily witnessed in this romantic country; of hundreds of these graceful youths, without a care to wrinkle, or a fear to disturb the full expression of pleasure and enjoyment that beams upon their faces -- their long black hair mingling with their horses' tails, floating in the wind, while they are flying over the carpeted prairie, and dealing death with their spears and arrows, to a band of infuriated buffaloes; or their splendid procession in a war-parade, arrayed in all their gorgeous colours and trappings, moving with most exquisite grace and manly beauty, added to that bold defiance which man carries on his front, who acknowledges no superior on earth, and who is amenable to no laws except the laws of God and honour.

In addition to the knowledge of human nature and of my art, which I hope to acquire by this toilsome and expensive undertaking, I have another in view, which, if it should not be of equal service to me, will be of no less interest and value to prosperity. I have, for many years past, contemplated the noble races of red men who are now spread over these trackless forests and boundless prairies, melting away at the approach of civilization. Their rights invaded, their morals corrupted, their lands wrested from them, their customs changed, and therefore lost to the world; and they at last sunk into the earth, and the ploughshare turning the sod over their graves, and I have flown to their rescue--not of their lives or of their race (for they are "doomed" and must perish), but to the rescue of their looks and their modes, at which the acquisitive world may hurl their poison and every besom of destruction, and trample them down and crush them to death; yet, phoenix-like, they may rise front the "stain on a painter's palette," and live again upon canvass, and stand forth for centuries yet to come, the living monuments of a noble race. For this purpose, I have designed to visit every tribe of Indians on the Continent, if my life should be spared; for the purpose of procuring portraits of distinguished Indians, of both sexes in each tribe, painted in their native costume; accompanied with pictures of their villages, domestic habits, games, mysteries, religious ceremonies, &c. with anecdotes, traditions, and history of their respective nations.

If I should live to accomplish my design, the result of my labours will doubtless be interesting to future ages; who will have little else left from which to judge of the original inhabitants of this simple race of beings, who require but a few years more of the march of civilization and death, to deprive them of all their native customs and character. I have been kindly supplied by the, Commander-in-Chief of the Army and the Secretary of War, with letters to the commander of every military post, and every Indian agent on the Western Frontier, with instructions to render me all the facilities in their power, which will be of great service to me in so arduous an undertaking. The opportunity afforded me by familiarity with so many tribes of human beings in the simplicity of nature, devoid of the deformities of art; of drawing fair conclusions in the interesting sciences of physiognomy and phrenology; of......... and customs, rites, ceremonies, &c.; and the opportunity of examining the geology and mineralogy of this western, and yet unexplored country, will enable me occasionally to entertain you with much new and interesting information, which I shall take equal pleasure in communicating by an occasional Letter in my clumsy way.