In the meantime, however, Beauregard loses no time in advantageously posting his troops. On the morning of the 18th of July, when the Union advance enters Centreville, he has withdrawn all his advanced brigades within the Rebel lines of Bull Run, resting them on the South side of that stream, from Union Mills Ford, near the Orange and Alexandria railroad bridge, up to the stone bridge over which the Warrenton Pike crosses the Run,--a distance of some six to eight miles. Between the Rebel left, at Stone Bridge, and the Rebel right, at Union Mills Ford, are several fords across Bull Run--the general course of the stream being from the North-West to South-East, to its confluence with the Occoquan River, some twelve miles from the Potomac River. Mitchell's Ford, the Rebel center, is about three miles to the South-West of, and about the same distance North-East from, Manassas Junction. But it may be well, right here, to locate all these fordable crossings of the rocky, precipitous, and well-wooded Bull Run stream, between the Stone Bridge and Union Mills Ford. Thus, half a mile below the Stone Bridge is Lewis's Ford; half a mile below that, Ball's Ford; half a mile below that, Island Ford; one and one-half miles below that, Mitchell's Ford--one mile below that. Blackburn's Ford; three-quarters of a mile farther down, McLean's Ford; and nearly two miles lower down the stream, Union Mills Ford. By Thursday morning, the 18th of July, Beauregard has advantageously posted the seven brigades into which he has organized his forces, at these various positions along his extended front, as follows: At the Stone Bridge, Brigadier-General N. G. Evans's Seventh Brigade, of one regiment and one battalion of Infantry, two companies of Cavalry, and a battery of four six-pounders. At Lewis's, Balls, and Island Fords--Colonel P. St. George Cocke's Fifth Brigade, of three regiments of Infantry, one battery of Artillery, and one company of Cavalry. At Mitchell's Ford, Brigadier-General M. L. Bonham's First Brigade, of four Infantry regiments, two batteries, and six companies of Cavalry. At Blackburn's Ford, Brigadier-General J. Longstreet's Fourth Brigade, of four Infantry regiments, with two 6-pounders. At McLean's Ford, Brigadier-General D. R. Jones's Third Brigade of three Infantry regiments, one Cavalry company, and two 6-pounders. At Union Mills Ford, Brigadier-General R. S. Ewell's Second Brigade, of three Infantry regiments, three Cavalry companies, and four 12-powder howitzers--Colonel Jubal A. Early's Sixth Brigade, of three Infantry regiments and three rifled pieces of Walton's Battery, being posted in the rear of, and as a support to, Ewell's Brigade. [Johnston also found, on the 20th, the Reserve Brigade of Brig. Gen. T. H. Holmes--comprising two regiments of Infantry, Walker's Battery of Artillery, and Scott's Cavalry-with Early's Brigade, "in reserve, in rear of the right."]