HEADQUARTERS WESTERN DEPARTMENT, Camp near Warsaw, Mo., October 16, 1861.

Brigadier General LORENZO THOMAS,
Adjutant-General of the Army, Washington, D. C.:

SIR: I have the honor to report that, after pushing the works of preparation at Saint Louis, Jefferson City, and Tipton, I left the latter place on the 14th instant, and, having fully matured my plans of operation against the enemy, have now reached this place with the major part of the division of Acting Major-General Asboth, near 5,000 strong, and only need the co-operation of the other divisions, already in march, or about to march, from their several points of departure on the line of the railroad, to move with all possible celerity against the enemy. Part of the division of Acting Major-General Sigel, which marched from Sedalia on the 13th, occupied Warsaw on the afternoon of the 15th, and this morning a detachment of cavalry, with the battalion of four companies of Missouri infantry, crossed the Osage River (not yet fordable). The other portions of his force have also been crossing during the day.

On the 14th the position of the enemy was reported as follows:

General Price, northeast of Montevallo, at the Prairie House; Steen's command, at Montevallo; Harrison's troops, at Big Alder Creek, and Rains' troops, at a place three miles below Montevallo, on the same stream.

After the crossing of the Osage by the cavalry this morning, Sergeant Trautweire reports that on their advance towards the Pomme de Terre River Bridge they had an engagement with some rebel cavalry, taking several prisoners. Further reports of my operations will be duly dispatched to the Department.

I have the honor to be, very respectfully, your obedient servant,

J. C. FREMONT,

Major-General, Commanding.

HEADQUARTERS, Saint Louis, Mo., October 16, 1861.

Colonel T. A. SCOTT,
Assistant Secretary of War, Washington, D. C.:

The effect of the special Washington dispatches to the New York Tribune on Missouri affairs has been to stimulate the rebels to great activity and aggression in the city and State. They are concentrating around Pilot Knob, having burned the bridge on the Iron Mountain road, to stop our re-enforcements. We have sent two regiments, and will whip them. They have cut the northern wires and are active and impudent. Hardee and Johnston have gone to Kentucky. General Fremont is 30 miles south of Tipton, marching fast. Your dispatch came while I was writing this. General Cameron left here Monday noon, he said, for Kentucky, to reach Washington Saturday night or Monday. I shall see you Monday or earlier.

B. RUSH PLUMLY,

Major, Acting Aide-de-Camp.