while days optically turbid may be acoustically clear. We have now to consider, in detail, the influence of various agents which have hitherto been considered potent in reference to the transmission of sound through the atmosphere.
Derham, and after him all other writers, considered that falling rain tended powerfully to obstruct sound. An observation on June 3d has been already referred to as tending to throw doubt on this conclusion. Two other crucial instances will suffice to show its untenability. On the morning of October 8th, at 7.45 a. m., a thunder-storm, accompanied by heavy rain, broke over Dover. But the clouds subsequently cleared away, and the sun shone strongly on the sea. For a time the optical clearness of the atmosphere was extraordinary, but it was acoustically opaque. At 2.30 p. m. a densely black scowl again overspread the heavens to the west-southwest. The distance being 6 miles, and all hushed on board, the horn was heard very feebly, the siren more distinctly, while the howitzer was better than either, though not much superior to the siren.
A squall approached us from the west. In the Alps or elsewhere I have rarely seen the heavens blacker. Vast cumuli floated to the northeast and southeast; vast streamers of rain descended in the west-northwest; huge scrolls of cloud hung in the north; but spaces of blue were to be seen to the north-northeast.
At 7 miles' distance the siren and horn were both feeble, while the guns sent us a very faint report. A dense shower now enveloped the Foreland.
The rain at length reached us; falling heavily all the way between us and the Foreland. But the sound, instead of being deadened, rose perceptibly in power. Hail was now added to the rain, and the shower reached a tropical violence, the hailstones floating thickly on the flooded deck. In the midst of this furious squall both the horns and the siren were distinctly heard; and as the shower lightened, thus lessening the local pattering, the sounds so rose in power that we heard them at a distance of 71⁄2 miles distinctly louder than they had been heard through the rainless atmosphere of 5 miles.
At 4 p. m. the rain had ceased, and the sun shone clearly through the calm air. At 9 miles' distance the horn was heard feebly, the siren clearly, while the howitzer sent us a loud report. All the sounds were better heard at this distance than they had previously been at 51⁄2 miles; from which, by the law of inverse squares, it follows that the intensity of the sound at 51⁄2 miles' distance must have been augmented at least threefold by the descent of the rain.
On the 23d of October, our steamer had forsaken us for shelter, and I sought to turn the weather to account by making other observations on both sides of the fog-signal station. Mr. Douglas, the chief-engineer of the Trinity House, was good enough to undertake the observations northeast of the Foreland; while Mr. Ayres, the
