You are no doubt inquiring, who are these Pawnees, Camanchees, and Arapahoes, and why not tell us all about them? Their history, numbers and limits are still in obscurity; nothing definite is yet known of them, but I hope I shall soon be able to give the world a clue to them. If my life and health are preserved, I anticipate many a pleasing scene for my pencil, as well as incidents worthy of reciting to the world, which I shall occasionally do, as opportunity may occur. LETTER—No. 40. MOUTH OF FALSE WASHITA. Since I wrote my last Letter from this place, I have been detained here with the rest of the cavalcade from the extraordinary sickness which is afflicting the regiment, and actually threatening to arrest its progress. It was, as I wrote the other day, the expectation of the commanding officer that we should have been by this time recruited and recovered from sickness, and ready to start again on our march; but since I wrote nearly one half of the command, and included amongst them, several officers, with General Leavenworth, have been thrown upon their backs, with the prevailing epidemic, a slow and distressing bilious fever. The horses of the regiment are also sick, about an equal proportion, and seemingly suffering with the same disease. They are daily dying, and men are calling sick, and General Leavenworth has ordered Col. Dodge to select all the men, and all the horses that are able to proceed, and be off to-morrow at nine o’clock upon the march towards the Camanchees, in hopes thereby to preserve the health of the men, and make the most rapid advance towards the extreme point of destination. General Leavenworth has reserved Col. Kearney to take command of the remaining troops and the little encampment; and promises Colonel Dodge that he will himself be well enough in a few days to proceed with a party on his trail and overtake him at the Cross Timbers.