448 SEC. 12. APPLIED MECHANICS. 



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. 



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 

 1'2-inch aperture had hitherto been the limit of size. 

 I 



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. An Improved Six-Concentric- wick Lamp, for 



burning 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.I.C.E., only the three outer wicks are to be used in ordinary 

 weather, and in thick weather the three inner wicks (at other times cut off by 

 a concentric reflector) are also brought into action. The full light-producing 

 power of the six-wick lamp is equal to 722 standard sperm candles, attained 

 by a consumption of 1 gallon of oil in 1 hour 50 minutes ; its half-power 

 equals 342 candles, with 1 gallon consumed in 2 hours 45 minutes. By a 

 simple arrangement regulating the level of oil in the burner, and the position 

 of the air-deflectors, the lamp can be made to burn any description of oil at 

 pleasure. 



2189. A Panel of Cata-Dioptric Apparatus, One of a set 

 of Polyzonal Lenses manufactured in 1836 by Messrs. Cookson 

 and Sons, of Newcastle, for the Trinity House of London, and 

 by them fixed in the Start Point Lighthouse, Devonshire. 



Trinity House, London. 



The first lenticular apparatus used in an English lighthouse, with a central 

 lamp upon Fresnel's principle. 



2190. Piano-Convex Lens 9 used at Portland Lighthouse 

 in the year 1789. Trinity House, London. 



It is 22 inches in diameter, and was placed in front of an argand burner 

 and reflector. It is believed to be one of the lenses first used in combination 

 with an oil lamp and reflector for lighthouse illumination. 



