Work on the tunnel continued steadily from June 25, and by July 17 the diggers were nearly 511 feet from the entrance and directly beneath the battery in Pegram’s Salient. The Confederates had learned of the mine by this time and had dug several countermines behind their own lines in an effort to locate the Union gallery. Two were very close, being dug on either side of where the Pennsylvanians were at work. Although digging in the countermines continued throughout July, Confederate fears seemed to lessen during the same period. There were many reasons for this, one being the failure of their tunnels to strike any Union construction. Another major reason, undoubtedly, was a belief held by many that it was impossible to ventilate a shaft of any length over 400 feet without constructing air shafts along it, and so far no air shafts could be seen between the Union and Confederate lines. The next step in the Union plan was to burrow out into lateral galleries at the end of the long shaft. Accordingly, on July 18, work was begun on these branches which extended to the right and left, paralleling the Confederate fortifications above. When completed, these added another 75 feet to the total length of the tunnel which now reached 586 feet into the earth. It was about 20 feet from the floor of the tunnel to the enemy works above. The average internal dimensions of the shaft were 5 feet high, with a base 4½ feet wide tapering to 2 feet at the top. Digging was finally completed on July 23. Four days later the task of charging the mine with black powder was accomplished. Three hundred and twenty kegs of powder weighing about 25 pounds each were arranged in the two lateral galleries in eight magazines. The total charge was 8,000 pounds. The powder was sandbagged to direct the force of the explosion upward and the fuses were spliced together to form a 98-foot line. Meanwhile, preparations for the large-scale attack which was to follow the explosion of the mine had been carried out. Burnside wanted his IX Corps to lead the attack, spearheaded by a fresh, 4,300-man Negro division, and pressed his wishes on Meade. Both Meade and Grant approved the request, but refused to allow the black troops to lead the assault for fear that, if the attack failed, the Union commanders could be accused of wanting to get rid of the only Negro troops then with the Army of the Potomac. Burnside did not learn of this decision until the day before the assault, July 29, and he was forced to change his plans at the last moment. Three white divisions would make the initial charge, with the black division in reserve. Burnside had the commanding generals of these three divisions draw straws to see which would lead. Brig. Gen. James F. Ledlie of the 1st Division won the draw.