690 THE POPULAR SCIENCE MONTHLY. 



sound through cambric, silk, lint, flannel, baize, and felt, the reader 

 is prepared for the statement that the sound-waves pass without sen- 

 sible impediment through heavy artificial showers of rain, hail, and 

 snow. Water-drops, seeds, sand, bran, and fiocculi of various kinds, 

 have been employed to form such showers : through all of these, as 

 through the actual rain and hail already described, and through the 

 snow on the Mer de Glace, the sound passes without sensible ob- 

 struction. 



Action of Fog : Obsebvations in London. But the mariner's 

 greatest enemy, fog, is still to be dealt with ; and here for a long time 

 the proper conditions of experiment were absent. Up to the end of 

 November we had had frequent days of haze, sufficiently thick to 

 obscure the white cliffs of the Foreland, but no real fog. Still those 

 cases furnished demonstrative evidence that the notions entertained 

 regarding the reflection of sound by suspended particles were wrong ; 

 for on many days of the thickest haze the sound covered twice the 

 range attained on other days of perfect optical transparency. Such 

 instances dissolved the association hitherto assumed to exist between 

 acoustic transparency and optic transparency, but they left the action 

 of dense fo<xs undetermined. 



On December 9th a memorable fog settled down on London. I 

 addressed a telegram to the Trinity House suggesting some gun-ob- 

 servations. With characteristic promptness came the reply that they 

 would be made in the afternoon at Blackwall. I went to Greenwich 

 in the hope of hearing the guns across the river ; but the delay of the 

 train by the fog rendered my arrival too late. Over the river the fog 

 was very dense, and through it came various sounds with great dis- 

 tinctness. The signal-bell of an unseen barge rang clearly out at 

 intervals, and I could plainly hear the hammering at Cubitt's Town, 

 half a mile away, on the opposite side of the river. No deadening of 

 the sound by the fog was apparent. 



Through this fog and various local noises, Captain Atkins and Mr. 

 Edwards heard the report of a 12-pounder cannonade with a 1-lb. 

 charge distinctly better than the 18-pounder with a 3-lb. charge, an 

 optically clear atmosphere, and all noise absent, on July 3d. 



Anxious to turn to the best account a phenomenon for which we 

 had waited so long, I tried to grapple with the problem by experi- 

 ments on a small scale. On the 10th I stationed my assistant with a 

 whistle and organ-pipe on the walk below the southwest end of the 

 bridge dividing Hyde Park from Kensington Gardens. From the 

 eastern end of the Serpentine I heard distinctly both the whistle and 

 the pipe, which produced 380 waves a second. On changing places 

 with my assistant, I heard for a time the distinct blast of the whistle 

 only. The deeper note of the organ-pipe at length reached me, rising 

 sometimes to great distinctness, and sometimes falling to inaudibility. 



