74 Royal Society : — Prof. Reynolds on 



extension which divergence would thus add to the range of the 

 sound would obviously depend on the refraction — that is to say, 

 when the direct rays were last refracted upwards, the extension of 

 the range due to divergence would be greatest. It is difficult to 

 say what the precise effect of this divergence would be ; but we may 

 assume that it would be similar to that which was found in the case 

 of wind, only the refraction being so much smaller the extension 

 of the range by divergence would be greater. On the whole the 

 results calculated from the data furnished by Mr. Glaisher agree 

 in a remarkable manner with those observed ; for if we add \ mile 

 for the extension of the range by divergence, the calculated dis- 

 tance with a clear sky would be two miles from a cliff 235 feet 

 high. — September 1874.] 



Now Prof. Tyndall found that from the cliffs at the South 

 Eoreland, 235 feet high, the minimum range of sound was a little 

 more than 2 miles, and that this occurred on a quiet July day 

 with hot sunshine. The ordinary range seemed to be from 3 to 5 

 miles when the weather was dull, although sometimes, particularly 

 in the evening, the sounds were heard as far as 15 miles. This 

 was, however, only under very exceptional circumstances. Prof. 

 Tyndall also found that the interposition of a cloud was followed 

 b} r an almost immediate extension of the range of the sound. I 

 extract the following passages from Prof. Tyndall's Eeport : — 



" On June 2 the maximum range, at first only 3 miles, after- 

 wards ran up to about 6 miles. 



" Optically, June 3 was not at all a promising day ; the clouds 

 were dark and threatening, and the air filled with a faint haze ; 

 nevertheless the horns were fairly audible at 9 miles. An exceed- 

 ingly heavy rain-shower approached us at a galloping speed. The 

 sound was not sensibly impaired during the continuance of the rain. 



"July 3 was a lovely morning : the sky was of a stainless blue, 

 the air calm, and the sea smooth. I thought we should be able to 

 hear a long way off. We steamed beyond the pier end and listened. 

 The steam clouds were there, showing the whistles to be active ; 

 the smoke-puffs were there, attesting the activity of the guns. 

 Nothing was heard. We went nearer ; but at two miles horns and 

 whistles and guns were equally inaudible. This, however, being 

 near the limit of the sound-shadow, I thought that might have 

 something to do with the effect, so we steamed right in front of 

 the station, and halted at 3| miles from it. Not a ripple nor 

 a breath of air disturbed the stillness on board, but we heard 

 nothing. There were the steam-puffs from the whistles, and we 

 knew that between every two puffs the horn-sounds were em- 

 braced, but we heard nothing. We signalled for the guns ; there 

 were the smoke-puffs apparently close at hand, but not the slightest 

 sound. It was mere dumb-show on the Foreland. We steamed 

 in to 3 miles, halted, and listened with all attention. Neither the 

 horns nor the whistles sent us the slightest hint of a sound. The 

 guns were again signalled for ; five of them were fired, some ele- 

 vated, some fired point-blank at us. Not one of them was heard. 

 We steamed in to two miles, and had the guns again fired : the 



