Transparency and Opacity of the Atmosphere. 381 



Dover side, and on the Sandwich side, with the same intensity, at 

 6 miles. 



• A gap (employed by the engineers in making arrangements for 

 pointing the syren in any required direction) here occurred in our 

 observations. They were resumed, however, on November 21, 

 when comparative experiments were made upon the gun and syren. 

 Both sources of sound, when employed as fog-signals, will not un- 

 frequently have to cover an arc of 180°; and it was desirable to 

 know with greater precision how the sound is affected by the direc- 

 tion in which the gun or syren is pointed. 



The gun, therefore, was in the first instance pointed on us and 

 fired, then turned and fired along a line perpendicular to that join- 

 ing us and it. There was a sensible, though small, difference be- 

 tween the sounds which reached us in the two cases. A similar 

 experiment was made with the syren ; and here the falling off 

 when the instrument was pointed perpendicular to the line joining 

 us and it was very considerable. This is what is to be expected ; 

 for the trumpet associated with the syren is expressly intended 

 to gather up the sound and project it in a certain direction, while 

 no such object is in view in the construction of the gun. The expe- 

 riments here referred to were amply corroborated by others made 

 on November 22 and 23. 



On both of these days the ' Gralatea's ' guns were fired to wind- 

 ward and to leeward. The aerial echoes in the latter case were di- 

 stinctly louder and longer than in the former. The experiment has 

 been repeated many times, and always with the same result. 



In front of the Cornhill Coastguard Station, and only 1| mile 

 from the Foreland, the syren, on the 21st, though pointed towards 

 us, fell suddenly and considerably in power. Before reaching 

 Dover Pier it had ceased to be heard. The wind w T as here against 

 the sound ; but this, though it contributed to the effect, could not 

 account for it ; nor could the proximity of the shadow account for 

 it. To these two causes must have been added an acoustically 

 flocculent, though optically transparent, atmosphere. The experi- 

 ment demonstrates conclusively that there are atmospheric and 

 local conditions which, when combined, prevent our most power- 

 ful instruments from making more than a distant approach to 

 the performance which writers on fog-signals have demanded of 

 them. 



On November 24 the sound of the syren, pointed to windward, 

 was compared at equal distances in front of and behind the instru- 

 ment. It was louder to leeward in the rear than at equal distances 

 to windward in front. Hence in a wind the desirability of point- 

 ing the instrument to windward. The whistles were tested this 

 day in comparison with the syren deprived of its trumpet. The 

 Canadian and the 8-inch whistles proved the most effective ; but 

 the naked syren was as well heard as either of them. As regards 

 opacity, the 25th of November almost rivalled the 3rd of July. 

 The gun failed to be heard at a distance of 2- 8 miles, and it yielded 

 only a faint crack at 2\ miles. 



