On the 10th of February I transmitted the following report of observations made by one of my assistants on Monday afternoon, 3 p.m., near Edwards Ferry:

Since my last observation I have discovered an increase of encampments in and about Leesburg, Va. They have commenced throwing up earth-works on the south side of Goose Creek and one mile and a half from the river.

No additional improvement has been made upon the old work that commands the ferry, and I think it is still unfinished.

The large fort west of Leesburg has been improved. It also appears that they have mounted some heavy guns. I could see no charge about the works south of Leesburg. (I should judge that these were entrenchments.)

There are two large encampments (new) on the road running to the west from Leesburg, near the large stone house, which is, I think, one mile from town; also an encampment in the woods south of the large fort and west of the two encampments near the stone bridge.

On the north and south side of Leesburg I noticed an increase of encampments close to the town.

In and around the large fort west of Leesburg there is, I think, a regiment.

On Goose Creek, about three miles from the river, there are some encampments. I could not tell how many there were, as they are partly concealed by the woods.

About five miles to the southeast of Goose Creek and one mile from the river I observed large quantities of smoke rising from the woods.

To the rear of Ball's Bluff I observed a small camp (two or three companies). Judging from the size and number of encampments, I should think there were from 10,000 to 12,000 troops opposite.

POOLESVILLE, February 21, 1862.

T. S. C. LOWE:

I should like the balloon to be put in readiness to make an ascension as early as possible.

JNO. SEDGWICK,

Brigadier-General, Commanding Division.

On the 1st of March, by request of General Heintzelman, I was ordered to take a balloon to Pohick Church, on the Occoquan River, and the following are some of the reports made at that time:

POHICK CHURCH, March 5, 1862.

Captain MOSES,

Assistant Adjutant-General, Fort Lyon, Va.:

Have just made two ascensions with the balloon. It is fully inflated, and will take up two persons with all the ropes. If to-morrow is a fine day it would be a good time for the general to go up. I can see camp-fires on the Occoquan.

T. S. C. LOWE,

Chief Aeronaut, U. S. Army.

MARCH 6, 1862-11 a.m.

Brigadier-General MARCY,

Chief of Staff, Army of the Potomac:

I made two ascensions last evening. Saw fires at Fairfax Station; some on the road near the Occoquan. This morning cavalry scouts are visible on this side of the Occoquan below Sandy Run. There are five large smokes on the other side of the Occoquan, commencing at the ford below Wolf Creek and extending to the Potomac. Judging from appearances, compared with General Hooker's division, I should think their force inferior to his. The balloon at Budd's Ferry has been up all the morning at the same time with me. If the force here could be advanced across Pohick Creek on the heights, I should have no difficulty in getting very near the exact number of the enemy, as well as all of their fortified places.

We could also signal from one balloon to the other, which would be of importance to me.

I have sent for the balloon at Poolesville. Please inform me where to station it.

Your obedient servant,

T. S. C. LOWE,

Chief Aeronaut.