One of the most remarkable features of the gallery was the method devised to supply the diggers at the end with fresh air. The longer the tunnel grew, the more serious became the problem of ventilation. It had been considered impossible to dig a tunnel for any considerable distance without spacing shafts at regular intervals in order to replace the polluted air with a fresh supply. This problem had been solved by the application of the simple physical principle that warm air tends to rise. Behind the Union picket line and to the right of the mine gallery, although connected with it, the miners dug a ventilating chimney. Between the chimney and the mine entrance they erected an airtight canvas door. Through that door and along the floor of the gallery there was laid a square wooden pipe. A fire was then built at the bottom of the ventilating shaft. As the fire warmed the air it went up the chimney. The draft thus created drew the bad air from the end of the tunnel where the men were digging. As this went out, fresh air was drawn in through the wooden pipe to replace it. Work on the tunnel had been continuously pushed from the start on June 25. By July 17 the diggers were nearly 511 feet from the entrance and directly beneath the battery in Elliott’s Salient. The Confederates had become suspicious by this time, for the faint sounds of digging could be heard issuing from the earth. Their apprehension took the form of countermines behind their own lines. Several of these were dug in an effort to locate the Union gallery. Two were very close, being sunk on either side of where the Pennsylvanians were at work. Although digging in the countermines continued throughout the month of July, Confederate fears seemed to quiet down during the same period. There were many reasons for this. One was 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. 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 in width tapering to 2 feet at the top.