Time passed. Mark became attached to General H----'s staff, and spent many months before Petersburg. It was there he performed an exploit which has remained legendary in the annals of the war. During a night attack on one of our batteries, the rebels had succeeded in spiking a siege-gun which commanded their position, while it protected our working parties of sappers. As soon as the piece was rendered useless, the enemy's sharp-shooters, swarming in rifle-pits close to our lines, seriously impeded further progress on our part. Vexed at this interruption, the commander called for volunteers to unspike the gun; but as this involved getting on the breech, and becoming a target for the foe while the work lasted, no one seemed willing to undertake it. In this dilemma, Mark, being known as a skilled machinist, was consulted; and after an examination, he reported in favor of the practicability of the job, while admitting the extreme peril attending it. Perhaps any intention of executing it would have been abandoned, had not a comment, made by one of the men to another, to the effect that it was easy for officers to set tasks for privates which they were not willing to do themselves, been accidentally overheard by Mark, which stung him immediately into offering to accomplish the hazardous feat. Armed with several well-tempered bits and a brace, he went forth at nightfall on his perilous errand. Straddling the breech of the monstrous cannon, and crouching as low as possible, with the brace against his chest, he plied the drill vigorously. Scarcely had he begun to work, when he was perceived by the vigilant rebel marksmen, who immediately opened fire. He could see the long rifle-pit, not a hundred yards distant, ablaze with the flash of fifty rifles, and feel the wind of their bullets as they whistled past him. Fortunately, favored somewhat by the obscurity, but far more by good luck, he remained unscathed, save by a skin-grazing touch. In fifteen minutes (it seemed to him an hour) the vent was clear; a primer and lanyard were then passed up to him, and these affixed, he slipped off the cannon as quick as possible. Seeing him drop, the rebels imagined they had shot him, and sent up a yell of exultation, which was suddenly checked as a discharge of grape from the liberated gun scattered death among them. Mark was not destined to escape entirely uninjured, for in his haste to get off the gun, and anxiety to avoid any danger from its recoil, he fell heavily, and was picked up with a dislocated shoulder. This accident, however, entailed but a short confinement, and he was soon able to be on duty again. Needless to add, that Mark received full meed of praise for his daring achievement, which furthermore earned him the grade of colonel.