Extract from Maj. Gen. George B. McClellan's report covering operations of the Army of the Potomac from July 27, 1861, to November 9, 1862.

SIR:
Immediately after being assigned to the command of the troops around Washington I organized a secret service force under Mr. E. J. Allen, a very experienced and efficient person. This force up to the time I was relieved from command was continually occupied in procuring from all possible sources information regarding the strength, positions and movements of the enemy. (Mr. Allan Pinkerton was the trustworthy and efficient chief of the secret service mentioned under the assumed name of E. J. Allen.)

All spies, "contrabands," deserters, refugees and many prisoners of war coming into our lines from the front were carefully examined, first by the outpost and division commanders, and then by my chief of staff and the provost-marshal-general. Their statements taken in writing, and in many cases under oath, from day to day for a long period previous to the evacuation of Manassas comprised a mass of evidence which by careful digests and collations enabled me to estimate with considerable accuracy the strength of the enemy before us

I am, sir, very respectfully, your obedient servant,