686 THE POPULAR SCIENCE MONTHLY. 



while days optically turbid may be acoustically clear. We have now 

 to consider, in detail, the influence of various agents which have hith- 

 erto been considered potent in reference to the transmission of sound 

 through the atmosphere. 



Derham, and after him all other writers, considered that fallinc 

 rain tended powerfully to obstruct sound. An observation on June 

 3d has been already referred to as tending to throw doubt on this con- 

 clusion. Two other crucial instances will suffice to show its untena- 

 bility. On the morning of October 8th, at 7.45 a. m., a thunder-storm, 

 accompanied by heavy rain, broke over Dover. But the clouds sub- 

 sequently 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 V 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 ti'opical 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 tliat we 

 heard them at a distance of 1% 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 

 5^ miles ; from which, by the law of inverse squares, it follows that 

 the intensity of the sound at 5^ miles' distance must have been aug- 

 mented 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 obser- 

 vations 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 



