HEADQUARTERS ARMY OF THE POTOMAC, Washington, D. C., March 8, 1862.

Major General GEORGE B. McCLELLAN,
Commanding Army of the Potomac:

GENERAL: I have the honor to report the following information relative to the forces and defenses of the Army of the Potomac obtained to this date, which has been extracted from current statements made here by spies, contraband, deserters, refugees, and rebel prisoners of war, in the order of time as hereinafter indicated, and which at the time of reception were made the subject of special reports to you. I have also appended to this report extracts from statements, and have made the same a part of this report, a varied summary of the rebel forces and defenses of the line of the Army of the Potomac, showing by different combinations about the probable number of these forces and the locality and strength of their defenses.

By reference to the summary of this report it will be seen that a medium estimate of the rebel army of the Potomac is 115,500, located as follows, viz: At Manassas, Centerville, Bull Ruin, Upper Occoquan, and vicinities, 80,000; at Brooke's Station, Dumfries, Lower Occoquan, and vicinities; 18,000; at Leesburg and vicinity, 4,500; in the Shenandoah Valley, 13,000.

Of the above-mentioned forces information has been received up to date, as shown by summary in this report, of the following specific organizations, viz: At Manassas, Centerville, Bull Run, Upper Occuquan, and vicinity, sixty-one regiments and one battalion infantry; eight regiments, one battalion,and seven independent companies cavalry and fifteen companies artillery. In the Shenandoah Valley, twelve regiments infantry; one regiment and is independent companies cavalry, and fifteen companies artillery. At Leesburg, four regiments infantry, one regiment militia, five independent companies cavalry, and one company artillery.

It is unnecessary for me to say that in the nature of the case, guarded as to rebels have ever been against the encroachment of spies and vigilant as they have always been to prevent information of their forces, movements, and designs from going beyond their lines, it has been impossible, even by the us of every resource at our command, to ascertain with certainty the specific number and character of their forces. It may, therefore, safely be assumed that in so large an army as our information shows them to possess very much of its composition and very many of its forces have not been specifically ascertained, which, added to those already known, would largely increase their number and considerably swell its proportions.

The summary of the general estimates shows the forces of the rebel Army of the Potomac to be 150,000, as claimed by its officers and sanctioned by the public belief, and that over 80,000 were stationed at Centerville, Manassas, and vicinity, the remainder being within easy supporting distance.

The statements of several reliable persons, who derived their information from the Commissary Department, show that in March, 1862, 80,000 daily rations were issued to the army at Manassas and Centerville; and the evidence is equally positive that each wing of the army, one in the Shenandoah Valley and the other on the Lower Potomac, had its sepa-