X. - LIGHTHOUSES, ETC. 535 



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' Mechanical Compound Reed Fog 

 Horn. JV. J, Holmes. 



These mechanical fog alarms arc 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, 

 by which means (the split tongue) a powerful vibration is set up with a 

 minimum pressure of air. The air bellows consist of two metal cylinders, 

 one working inside the other ; and the compressed air upon the return of 

 the cylinder is driven through the reed into an inner trumpet-shaped tube 

 contained within and a part of the external cylinder. 



2185c. Drawing of the Steam Syren or Fog 



erected on the South Foreland, coast of Kent. 



Prof. W. F. Barrett. 



2186. Parabolic Reflector of 21-inch aperture. 



Trinity House, London. 



Composed of copper coated with pure silver in the proportion of 3 ozs. 

 (troy) pure silver to 1 Ib. (avoirdupois) copper. Its focal distance is 3 

 inches. 



Improvements in the construction of light vessels' lanterns have permitted 

 the introduction of this large sized reflector into that service, in which a 

 12-inch aperture had hitherto been the limit of size. 



2187. Centrifugal Governor. A mechanical arrangement 

 used for controlling the movements of the clockwork machinery, 

 by means of which a light is made to revolve on board a light 

 ship. Trinity House, London. 



Before this contrivance was designed by Mr. Slight of the Trinity Work- 

 shops, there was always a tendency to irregularity in the periods of duration 

 of the light and the interval of darkness, but the centrifugal governor ensures 

 the working of the revolving machinery with very great accuracy. 



2188. Improved Six-Concentric-wick Lamp, for burn- 

 ing vegetable or mineral oil. Trinity House, London. 



The burner hitherto in use for dioptric apparatus of the first order has 

 carried four concentric wicks, and in burning has been maintained at full 

 power in all weathers. In the improved six-wick burner, designed by J. N. 

 Douglass, M.Inst. C.E., only the three outer wicks are to be used in ordinary 



