The Forty-third Illinois, of McClernand's brigade, being out by permission, Sunday morning, to discharge their pieces, which had been loaded since they marched to the picket-line, Friday evening, distant firing was heard. This being reported to General McClernand, he sent an order to Colonel Reardon to hold the brigade in readiness for action. Colonel Reardon, being confined to bed by illness, directed Colonel Raith to assume command. There was some delay in getting the brigade formed, owing to the sudden change of commanders and to the incredulity of the officers in some of the regiments as to the reality of an attack. The brigade being at length formed, advanced, and took position, with its right near Waterhouse's battery--its line making an angle with Sherman's line, so as to throw the left of the brigade upon and along Oak Creek. Colonel Marsh, of the Twentieth Illinois, heard considerable musketry on the left of the National camp. This continuing without material interruption for some time, he ordered regimental commanders to be in readiness to form, and soon after received an order from General McClernand to form the brigade. Soon after the brigade was formed an order was received to advance to the support of General Sherman, who was reported to be heavily attacked. The brigade moved to the left to a position assigned by General McClernand. The First Brigade was ordered to form three regiments on the left of the Second, and to post one regiment, the Eleventh Iowa, in reserve in rear of the right of Colonel Marsh's brigade. The alignment of the Third Brigade, by Colonel Raith throwing his left too far to the front, so as to be exposed to a flank attack and also to cover Colonel Marsh's right, Colonel Raith wheeled his left to the rear to connect with Marsh. The right of McClernand's division, as thus formed, connected with Sherman, but the left was uncovered.