THE ATMOSPHERE AND FOG-SIGNALING. 543 



deadening power of fog than the paper of Derham, published one hun- 

 dred and sixty-seven years ago. In consequence of their a priori 

 probability, his conclusions seem to have been transmitted unques- 

 tioned from generation to generation of scientific men. 



Instruments and Observations. On the 19th of May, 1873, this 

 inquiry began. The South Foreland, near Dover, was chosen as the 

 signal-station, steam-power having been already established there to 

 work two powerful magneto-electric lights. The observations were 

 mostly made afloat, one of the yachts of the Trinity Corporation being 

 usually employed for this purpose. Two stations had been established, 

 one at the top, the other at the bottom, of the South-Foreland Cliff; and, 

 at each, trumpets, air-whistles, and steam whistles of great size, were 

 mounted. The whistles first employed were of English manufacture. 

 To these were afterward added a large United States whistle, also a 

 Canadian whistle, of great reputed power. 



On the 8th of October another instrument, which has played a 

 specially important part in these observations, was introduced. This 

 was a steam-siren, constructed and patented by Mr. Brown, of New 

 York, and introduced by Prof. Henry into the light-house system of 

 the United States. As an example of international courtesy worthy 

 of imitation, I refer with pleasure to the fact that, when informed by 

 Major Elliott, of the United States Army, that our experiments had 

 begun, the Light-house Board at Washington, of their own spontaneous 

 kindness, forwarded to us for trial a very noble instrument of this de- 

 scription, which was immediately mounted at the South Foreland. 



The principle of the siren is easily understood. A musical sound 

 is produced when the tympanic membrane is struck periodically with 

 sufficient rapidity. The production of these tympanic shocks by puffs 

 of air was first realized by Dr. Robinson, and his device was the first 

 and simplest form of the siren. A stopcock was so constructed that 

 it opened and shut the passage of a pipe 720 times in a second. Air 

 from the wind-chest of an organ being allowed to pass along the pipe 

 during the rotation of the cock, a musical sound was most smoothly 

 uttered. A great step was made in the construction of the instrument 

 by Cagniard de la Tour, who gave it its present name. He employed 

 a box with a perforated lid, and above the lid a similarly perforated 

 disk capable of rotation. The perforations were oblique, so that when 

 wind was driven through the lid, it so impinged upon the apertures 

 of the disk as to set it in motion. No separate mechanism was there- 

 fore required to turn the disk. When the perforations of lid and disk 

 coincided, a puff escaped ; when they did not coincide, the current of 

 air was cut off. In this way impulses were imparted to the air, and 

 sound-waves generated. The siren has been greatly improved by 

 Dove, and specially so by Helmholtz. Even in its small form, it can 

 produce sounds of great intensity. 



