1874.] Transmission of Sound by the Atmosphere. 65 



reached on the bearing of the Varne light- vessel to a distance of 11 J 

 miles. The syren was heard through the paddle-noises at 9| miles, 

 while at 8 1 miles it became efficient as a signal with the paddles going. 

 The horns were heard at 6| miles. This was during calm. Subsequently, 

 with a wind from the N.N.W., no sounds were heard at 6| miles. On 

 land, the wind being across the direction of the sound, the syren was 

 heard only to a distance of 3 miles N.E. of the Foreland ; in the other 

 direction it was heard plainly on Folkestone Pier, 8 miles distant. 

 Both gun and horns failed to reach Folkestone. 



Wind, rain, a rough sea, and great acoustic opacity characterized 

 October 31. Both gun and horns were unheard 3 miles away, the 

 syren at the same time being clearly heard. It afterwards forced its 

 sound with great power through a violent rain-squall. Wishing the 

 same individual judgment to be brought to bear upon the sounds on both 

 sides of the Foreland, in the absence of our steamer, which had quitted 

 us for safety, I committed the observations to Mr. Douglass. He heard 

 them at 2 miles on the 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 ex- 

 periments were made upon the gun and syren. Both sources of sound, 

 when employed as fog-signals, will not (infrequently have to cover an arc 

 of 180 ; and it was desirable to know with greater precision how the 

 sound is affected by the direction 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 joining us and it. 

 There was a sensible, though small, difference between 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 perpen- 

 dicular 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 experi- 

 ments here referred to were amply corroborated by others made on 

 November 22 and 23. 



On both of these days the * Galatea's ' guns were fired to windward 

 and to leeward. The aerial echoes in the latter case were distinctly 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 1J 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 was here against the sound ; but this, 

 though it contributed to the effect, could not account for it, nor could the 



