Page:Popular Science Monthly Volume 6.djvu/566

548 An interval of 12 hours sufficed to change in a surprising degree the acoustic transparency of the air. On the 1st of July the sound had a range of nearly 13 miles; on the 2d the range did not exceed 4 miles.

—Thus far the investigation proceeded with hardly a gleam of a principle to connect the inconstant results. The distance reached by the sound on the 19th of May was 3$1/2$ miles; on the 20th it was 5$1/2$ miles; on the 2d of June, 6 miles; on the 3d, more than 9 miles; on the 10th it was also 9 miles; on the 25th it fell to 6$1/2$ miles; on the 26th it rose again to more than 9$1/4$ miles; on the 1st of July, as we have just seen, it reached 12$3/4$, whereas on the 2d the range shrunk to 4 miles. None of the meteorological agents observed could be singled out as the cause of these fluctuations. The wind exerts an acknowledged power over sound, but it could not account for these phenomena. On the 25th of June, for example, when the range was only 6$1/2$ miles, the wind was favorable; on the 26th, when the range exceeded 9$1/4$ miles, it was opposed to the sound. Nor could the varying optical clearness of the atmosphere be invoked as an explanation; for, on July 1st, when the range was 12$3/4$ miles, a thick haze hid the white cliffs of the Foreland, while on many other days, when the acoustic range was not half so great, the atmosphere was optically clear. Up to July 3d all remained enigmatical; but on this date observations were made which seemed to me to displace surmise and perplexity by the clearer light of physical demonstration.

—On July 3d we first steamed to a point 2.9 miles southwest-by-west of the signal station. No sounds, not even the guns, were heard at this distance. At two miles they were equally inaudible. But this being a position at which the sounds, though strong in the axis of the horn, invariably subsided, we steamed to the exact bearing from which our observations had been made on July 1st. At 2.15, and at a distance of 3$3/4$ miles from the station, with calm, clear air and a smooth sea, the horns and whistle (American) were sounded, but they were inaudible. Surprised at this result, I signaled for the guns. They were all fired, but, though the smoke seemed at hand, no sound whatever reached us. On July 1st, in this bearing, the observed range of both horns and guns was 10$1/2$ miles, while, on the bearing of the Varne light-vessel, it was nearly 13 miles. We steamed in to 3 miles, paused, and listened with all attention; but neither horn nor whistle was heard. The guns were again signaled for; five of them were fired in succession, but not one of them was heard. We steamed on in the same bearing to 2 miles, and had the guns fired point-blank at us. The howitzer and the mortar, with 3-lb. charges, yielded a feeble thud, while the 18-pounder was wholly unheard. Applying the law of inverse squares, it follows that, with the air and sea, according to accepted notions, in a far worse