THE ATMOSPHERE AND FOG-SIGNALING. 551 



mile nearer. "We steamed out to 4^ miles, where the sounds were for 

 a moment faintly heard ; but they fell away as we waited, and though 

 the greatest quietness reigned on board, and though the sea was with- 

 out a ripple, we could hear nothing. We could plainly see the steam- 

 putt's which announced the beginning and the end of a series of trum- 

 pet-blasts, but the blasts themselves were qnite inaudible. 



It was now 4 p. m., and my intention at first was to halt at this dis- 

 tance, which was beyond the sound-range, but not far beyond it, and 

 see whether the lowering of the sun would not restore the power of 

 the atmosphere to transmit the sound. But, after waiting a little, the 

 anchoring of a boat was suggested, so as to liberate the steamer for 

 other work ; and, though loath to lose the anticipated revival of the 

 sounds myself, I agreed to this arrangement. Two men were placed 

 in the boat and requested to give all attention, so as to hear the sound 

 if possible. With perfect stillness around them, they heard nothing. 

 They were then instructed to hoist a signal if they should hear the 

 sounds, and to keep it hoisted as long as the sounds continued. 



At 4.45 we quitted them and steamed toward the South Sand Head 

 light-ship. Precisely fifteen minutes after we had separated from them 

 the flag was hoisted : the sound had at length succeeded in piercing 

 the body of air between the boat and the shore. 



We continued our journey to the light-ship, went on board, heard 

 the report of the lightsmen, and returned to our anchored boat. We 

 then learned that when the flag was hoisted the horn-sounds were 

 heard, that they were succeeded after a little time by the whistle- 

 sounds, and that both increased in intensity as the evening advanced. 

 On our arrival, of course we heard the sounds ourselves. 



We pushed the test further by steaming farther out. At 5f miles, 

 we halted and heard the sounds; at 6 miles we heard them distinctly, 

 but so feebly that we thought we had reached the limit of the sound- 

 range ; but while we waited the sounds rose in power. We steamed 

 to the Varne buoy, which is 7f miles from the signal-station, and 

 heard the sounds there better than at 6 miles' distance. We con- 

 tinued our course outward to 10 mftes, halted there for a brief inter- 

 val, but heard nothing. 



Steaming, however, on to the Varne light-ship, which is situated 

 at the other end of the Varne shoal, we hailed the master, and were 

 informed by him that up to 5 p. m. nothing had been heard, but that 

 at that hour the sounds began to be audible. He described one of 

 them as " very gross, resembling the bellowing of a bull," which very 

 accurately characterizes the sound of the large American steam-whistle. 

 At the Varne light-ship, therefore, the sounds had been heard toward 

 the close of the day, though it is 12f miles from the signal-station. I 

 think it probable that, at a point 2 miles from the Foreland, the sound 

 at 5 P. m. possessed fifty times the intensity which it possessed at 2 

 p. m. To such undreamt-of fluctuations is the atmosphere liable. On 



