We pushed the test further by steaming further out. At 5-3/4 miles we halted and heard the sounds: at 6 miles we heard them distinctly, but so feebly that we thought we had reached the limit of the sound-range; but while we waited the sounds rose in power. We steamed to the Varne buoy, which is 7-3/4 miles from the signal-station, and heard the sounds there better than at 6 miles’ distance. We continued our course outward to 10 miles, halted there for a brief interval, but heard nothing. Steaming, however, on to the Varne light-ship, which is situated at the other end of the Varne shoal, we hailed the master, and were informed by him that up to 5 P.M. nothing had been heard, but that at that hour the sounds began to be audible. He described one of them as “very gross, resembling the bellowing of a bull,” which very accurately characterizes the sound of the large American steam-whistle. At the Varne light-ship, therefore, the sounds had been heard toward the close of the day; though it is 12-3/4 miles from the signal-station. I think it probable that, at a point 2 miles from the Foreland, the sound at 5 P.M. possessed fifty times the intensity which it possessed at 2 P.M. To such undreamed-of fluctuations is the atmosphere liable. On our return to Dover Bay, at 10 P.M., we heard the sounds, not only distinct but loud, where nothing could be heard in the morning. § 5. _Other Remarkable Instances of Acoustic Opacity_ In his excellent lecture entitled “Wirkungen aus der Ferne,” Dove has collected some striking cases of the interception of sound. The Duke of Argyll has also favored me with some highly-interesting illustrations; but nothing of this description that I have read equals in point of interest the following account of the battle of Gaines’s Farm, for which I am indebted to the Rector of the University of Virginia: “LYNCHBURG, VIRGINIA, _March 19, 1874_. “SIR—I have just read with great interest your lecture of January 16th, on the acoustic transparency and opacity of the atmosphere. The remarkable observations you mention induce me to state to you a fact which I have occasionally mentioned, but always, where I am not well known, with the apprehension that my veracity would be questioned. It made a strong impression on me at the time, but was an insoluble mystery until your discourse gave me a possible solution.