SAINT LOUIS ARSENAL, July 6, 1861.

General McCLELLAN,

Buckhannon, Va.:

General Lyon has sent Wyman's regiment to southwest. This, with the 700 troops now there, will be enough for the present. Colonel Wyman is in command, with instructions to keep open the line of communication, on which all supplies will be sent hereafter. General Lyon has moved down towards Springfield with 2,400 men, and Major Sturgis with 2,200 on the frontier. Sweeny is there and at Mouth Vernon, beyond there, with 2,500, besides guards at posts on lines. Marsh's Alton regiment is here. I will equip them. They will go to Cape Girardeau and be subject to General Prentiss' call in case of necessity. The Quincy regiment will go to Ironton, and thence to Greenville. I will write particulars to-night. Think the force sufficient, and will not order more unless necessary.

CHESTER HARDING, Jr.,

A. A. G., Missouri Volunteers.

SAINT LOUIS ARSENAL, July 7, 1861.

L. THOMAS,

Adjutant-General, Washington:

SIR: At the suggestion of General Lyon I write to inform you of the movements of troops in this State.

Three columns are in the field, with the design to unite at or near Springfield, Mo., and thence to proceed into Arkansas. General Lyon's intention was to go to Little Rock, but movements of the enemy in the southeastern portion of the State may change his plans. Besides garrisoning Jefferson City, Booneville, and Lexington, General Lyon has marched southward with 2,400 men, in round numbers. There are at Springfield and Mount Vernon, and on the way there from Rolla, about 3,000 men, under the command of Captain T. W. Sweeny, Second Infantry, acting (under election and by order from Brigadier-General Harney) as brigadier-general of the U. S. Reserve Corps of Saint Louis. In addition to these there are about 1,000 of the Home Guard and Rifle Battalion protecting the line of communication from Saint Louis to Springfield. As this line has become the most important one in the whole State, and as it is threatened by hostile bands under General McBride and others, it has been deemed best to place it under the command of Colonel Wyman, Thirteenth Illinois Volunteers, who went down to Rolla with his regiment last night. He will establish his headquarters either at Rolla or Lebanon, beyond the crossings of the Gasconade River, as he finds most expedient.

Colonel Marh's Twentieth Illinois Volunteers are now at this point to be equipped. After being fitted for field service they will move down to Cape Girardeau, within 50 miles of Cairo, where they will, in case of necessity, by subject to the orders of General Prentiss, but if not called for at that point, will stop the transportation of arms, munitions, and supplies which has been carried on between New Madrid and Cape Girardeau, and break up the rebel camps that have been formed in the vicinity.

General McClellan has placed at the disposal of General Lyon one of the regiments at Quincy. Orders have been sent for it to come here, where it will be equipped, and then sent down the Iron Mountain Railroad to Ironton, from whence it will proceed to Greenville, in Wayne County. Five companies of the Sixth Regiment U. S. Volunteers