The Photographic History of The Civil War: in ten volumes, Thousands of Scenes Photographed 1861-65, with Text by many Special Authorities, Volume 2: Two Years of Grim War.
Francis Trevelyan Miller, Ed.

Unmindful of the numbers which opposed him, General Hardee not only succeeded in repulsing the attack, but, assuming the offensive, drove back the forces under General John E. Smith, who had sought to turn his left, and captured several hundred prisoners. The Federals, quickly re-forming their lines, renewed the assault and for several hours the fighting was desperate on both sides. A General advance of the Northern forces had been withheld, awaiting the arrival of Hooker who, under orders from Grant, was sweeping down Chickamauga Valley, and was to operate against the Confederate left and rear, in the expectation that Bragg would further weaken his line by massing at those points. But Hooker's Army had been delayed several hours by repairs to the bridge crossing Chattanooga Creek. Although Sherman had failed in his attempt to turn the Confederate right he had forced Bragg to draw heavily upon his center for reenforcements. Grant, satisfied that Hooker was not far off, ordered the signal-six guns fired in rapid succession from the battery on Orchard Knob — for a General advance of Thomas' Army upon the Confederate center.

it was now three o'clock in the afternoon. The four division commanders of the Army of the Cumberland, Sheridan, Wood, Baird, and Johnson, gave the word to advance. Between Orchard Knob and the base of Missionary Ridge, a mile away, is a broad Valley covered for the most part with heavy timber. This had to be crossed before the entrenchments at the foot of the hill could be assaulted. Scarcely were the Cumberland troops in motion when fifty pieces of artillery on the crest of Missionary Ridge opened a terrific fire upon them. But the onward rush of the Federals was not checked in the slightest degree. The line of entrenchments at the base was carried with little opposition. Most of Breckinridge's men abandoned the ditches as the Federal skirmishers approached