Important from tlie Ilebel Army.A young man Darned William West, asun of a clerk in the Treasury Department, and who was pressed into the rebel ranks at Winchester, arrived at our lines Mouday morning. fie left Ontreville on Saturday, and makes the following report: The rebel forces near that point numberabout sixty thousand, being about one-half of their army of the Potomac. They are concentrated between Centreville and the Bull Ruu battlefield, in the angle formed by Bradduek’s Road and the roads leading to Manassas Junction. They haveabout fifteen batteries light artillery, but no Beige pieces, and four or five regiments of cavalry Their fortifications are small, insignificant, compared with ours, apparently merely breastworks for infantry,and do not extend ibis side of Centreville. The men are well clothed in homespun,have sufficient food and drink, though no coffee or sugar, and huve not yet Buffered from cold. The toots leak in a heavyram. Many Maryland and Virginia soldiers would gladly desert, but fear they would be ill-treated by our authorities — A proclamation that they would be kindly received would induce large numbers to come within our lines. On hearing of the National victory at Beaufort, many from South Carolina and Georgia were anxious to refurn home, but they are now satisfied to remain. Several of our soldiers who have been taken prisoners had enlisted in the rebel ranks, among them two of the foraging party captured a few days ago. Having to go to Richmond or | enlist, they chose the course that promised an early chance of escape. West w s an orderly of Gen. Espy, aud he effected ! his escape the more easily by reason of having a horse. The rebels had recently received a quantity of English muskets, were well disciplined, and had great confidence in their officers.«v