VIII. LIGHTHOUSES, ETC. 447 



The disc syren, with the trumpet by which its sound is directed, is shown 

 in the diagram. It is composed of a fixed disc, forming one end of the 

 chamber into which steam or compressed air is forced, and a movable disc 

 rotating rapidly by separate mechanism outside it. Both are perforated by 

 12 radial slits exactly corresponding each to each, and the rotation of the 

 moving disc, close to and upon a common axis with its fixed associate, 

 permits the compressed air or steam to escape when the slits coincide, and 

 shuts it off when they do not. The vibrations thus produced being repeated 

 in the instrument described, at the rate of more than 400 per second, emit a 

 sound of very great intensity, which is directed by the trumpet towards any 

 desired point. 



The cylindrical syren is a later form of the instrument, in which the 

 chamber for compressed air surrounds a fixed cylinder having 24 slits, within 

 which another cylinder coincidently perforated rotates, and the vibrations 

 pass through the open end of the inner cylinder to the trumpet. 



Syrens sounded by steam have for some time been used for fog-sig-' 

 nailing on the coasts of America, and have lately been adopted, with the 

 substitution of compressed air for steam, in Great Britain as a result of expe- 

 riments made by the Trinity House with the assistance of Professor Tyndall 

 during the winter of 1873. 



2185. Fog Signal Apparatus. Designed by Dr. G. Amadi, 

 of Trieste. 



The Imperial and Royal Maritime Government at Trieste. 



By this apparatus deep tones, like those of an organ, are produced by 

 metallic tongues, driven by steam, and sent through a trumpet in a given 

 direction. These, from experiments, have extended as far as 16 nautical, or 

 nearly four German miles. 



In working this apparatus, of which already three are in use, at Trieste, 

 Salvore, and Grado, the sounds are made self-producing at certain intervals 

 by means of a steam-engine. 



2185a. Holmes 9 Shipwreck Distress Signal Flare and 

 Life Buoy Rescue Lights. 



These have the remarkable property of bursting into flame when placed in 

 contact with water, and when once ignited are absolutely inextinguishable by 

 either wind or water. They emit a most powerful white light, as brilliant as 

 the magnesium light, and continue to burn over 30 minutes. The shipwreck 

 distress signal flare is visible on a dark night with a clear atmosphere at a 

 sufficient elevation for over ten nautical miles, and burns with greater 

 brilliancy the more seas sweep over it. 



The light is a chemical light, and produced by the action of the water upon 

 phosphuret of calcium, giving off phosphuret of hydrogen, which, combining 

 with the oxygen in the atmosphere, spontaneously ignites. These distress 

 signals are free from danger, are not affected by heat, friction, or percussion, 

 and contain no explosive compound whatever. 



2185b. Holmes 9 Mechanical Compound Heed Fog 

 Horn. 



These mechanical fog alarms are constructed upon the most approved 

 acoustical principles, and emit a most powerful sound. The " aurora " fog 

 horn can be heard over three nautical miles, and the note produced is the 

 8 foot'C of the musical scale. The tone is produced by the vibrations of two 

 metal tongues, placed together in absolute contact, and closing the same reed, 



