Page:Popular Science Monthly Volume 6.djvu/561

Rh deadening power of fog than the paper of Derham, published one hundred and sixty-seven years ago. In consequence of their a priori probability, his conclusions seem to have been transmitted unquestioned from generation to generation of scientific men.

—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 description, 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 therefore 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.