692 THE POPULAR SCIENCE MONTHLY. 



western sky. At 10.30 I sent an assistant to the bridge, and listened 

 for his whistle and pipe at the eastern end of the Serpentine. The 

 whistle rose to a shrillness far surpassing any thing previously heard> 

 but it sank sometimes almost to inaudibility ; provfng that, though 

 the air was on the whole highly homogeneous, acoustic clouds still 

 drifted through the fog. A second pipe, which was quite inaudible 

 yesterday, was plainly heard this morning. We were able to dis- 

 course across the Serpentine to-day with much greater ease than yes- 

 terday. 



During our summer observations, I had once or twice been able to 

 iix the position of the Foreland in thick haze by the direction of the 

 sound. To-day my assistant, hidden by the fog, walked up to the 

 Watermen's Boat-House sounding his whistle ; and I walked along 

 the opposite side of the Serpentine, clearly appreciating for a time that 

 the line joining us was oblique to the axis of the river. Coming to a 

 point which seemed to be exactly abreast of him, I marked it, and on 

 the following day, when the fog had cleared away, the marked po- 

 sition was found to be perfectly exact. When undisturbed by echoes, 

 the ear, with a little practice, becomes capable of fixing with great 

 precision the direction of a sound. 



On reaching the Serpentine this morning, a peal of bells, which 

 then began to ring, seemed so close at hand that it required some 

 reflection to convince me that they were ringing to the north of Hyde 

 Park. The sounds fluctuated wonderfully in power. Prior to the 

 striking of eleven by the great bell of Westminster, a nearer bell struck 

 with loud clangor. The first five strokes of the Westminster bell were 

 afterward heard, one of them being extremely loud ; but the last six 

 strokes were inaudible. An assistant was stationed to attend to the 

 twelve-o'clock bells. The clock which had struck so loudly at eleven 

 was unheard at twelve, while of the Westminster bell eight strokes 

 out of twelve were inaudible. To such astonishing changes is the 

 atmosphere liable. 



At 1 p. M. the Westminster bell striking seven was not at all heard 

 from the Serpentine, while the nearer bell already alluded to was 

 heard distinctly. The fog had cleared away, and the lamps on the 

 bridge could be seen from the eastern end of the Serpentine burning 

 brightly ; but, instead of the sound sharing the improvement of the 

 light, what might be pi-operly called an acoustic fog took the place 

 of its optical predecessor. Several series of the whistle and organ- 

 pipe were sounded in succession ; one series only of the whistle-sounds 

 was heard, all the others being quite inaudible. Three series of the 

 organ-pipe were heard, but exceedingly faintly. On reversing the 

 positions and sounding as before, nothing whatever was heard. 



At eight o'clock the chimes and hour-bell of the Westminster 

 clock were both very loud. The " acoustic fog " had shifted its po- 

 sition or temporarily melted away. 



