“On the afternoon of June 28, 1862, I rode, in company with General G. W. Randolph, then Secretary of War of the Confederate States, to Price’s house, about nine miles from Richmond; the evening before General Lee had begun his attack on McClellan’s army, by crossing the Chickahominy about four miles above Price’s, and driving in McClellan’s right wing. The battle of Gaines’s Farm was fought the afternoon to which I refer. The valley of the Chickahominy is about one mile and a half wide from hilltop to hilltop. Price’s is on one hilltop, that nearest to Richmond; Gaines’s farm, just opposite, is on the other, reaching back in a plateau to Cold Harbor. “Looking across the valley I saw a good deal of the battle, Lee’s right resting in the valley, the Federal left wing the same. My line of vision was nearly in the line of the lines of battle. I saw the advance of the Confederates, their repulse two or three times, and in the gray of the evening the final retreat of the Federal forces. “I distinctly saw the musket-fire of both lines, the smoke, individual discharges, the flash of the guns. I saw batteries of artillery on both sides come into action and fire rapidly. Several field-batteries on each side were plainly in sight. Many more were hid by the timber which bounded the range of vision. “Yet looking for nearly two hours, from about 5 to 7 P.M. on a midsummer afternoon, at a battle in which at least 50,000 men were actually engaged, and doubtless at least 100 pieces of field-artillery, through an atmosphere optically as limpid as possible, _not a single sound of the battle_ was audible to General Randolph and myself. I remarked it to him at the time as astonishing. “Between me and the battle was the deep broad valley of the Chickahominy, partly a swamp shaded from the declining sun by the hills and forest in the west (my side). Part of the valley on each side of the swamp was cleared; some in cultivation, some not. Here were conditions capable of providing several belts of air, varying in the amount of watery vapor (and probably in temperature), arranged like laminæ at right angles to the acoustic waves as they came from the battlefield to me. “Respectfully, “Your obedient servant, “R. G. H. KEAN. “PROF. JOHN TYNDALL.” I learn from a subsequent letter that during the battle the air was still.—J. T. § 6. _Echoes from Invisible Acoustic Clouds_