532 SEC. 12. APPLIED MECHANICS. 



X. LIGHTHOUSES AND FOG SIGNALS. 



2181. Model of a Lighthouse built upon the Bishop 

 Rock, 7 miles from land, forming part of the outermost reef 

 S.W. of the Scilly Islands. Trinity House, London. 



The tower is of Cornish granite (Carnsew), and is surmounted by a lantern 

 of gun-metal, containing lenticular apparatus to exhibit a fixed light of the 

 first order, whose focal plane is 110 feet above high-water spring tides. The 

 structure measures from base to vane 147 feet. It was built from designs by 

 the late James Walker, M.I.C.E., under the superintendence of Nicholas 

 Douglass, for the Corporation of Trinity House, London, at a cost of 36,500/., 

 and occupied six years in construction ; it was completed in 1853. A sectional 

 drawing shows the method of bracing by vertical and radiating wrouglit- 

 iron ties, lately adopted for strengthening the structure. 



2181a. Model of the Old Lighthouse on the Smalls 



Hocks, about 17 miles off the coast of Pembrokeshire. Erected 

 1776; replaced in 1861 by a stone lighthouse. The model is 

 made out of one of the oak pillars of the original lighthouse. 

 Executors of the late Captain Pickering Clarke, R.N. 



South Kensington Museum. 



2181b. Model of the Lighthouse on the Needles 



Bocks, Isle of Wight. Built 1857-1858. Light shown 1 January 

 1859. Trinity House, London. 8outh Kensington Museum. 



2181c. Model of the Gunneet Lighthouse. Iron. Built 

 on Mitchell's patent screw piles, of which a model is also shown. 

 The piles screw 40 ft. into the sand, and have screws 4 ft. in 

 diameter. James Walker, engineer. The Trinity House. 



South Kensington Museum. 



2181d. Series of Models, illustrating mark buoys, used by 

 the Trinity House Corporation round the British coast. The 

 Trinity House. , South Kensington Museum. 



2181e. Harbour Light. Chance's patent dioptric lens of 

 the fourth order, for fixed light. Chance Brothers and Company, 

 Glass Works, Birmingham. South Kensington Museum* 



21Slf. Babb age's occulting Light Apparatus. De- 

 signed by the late Charles Babbage, F.K.S., for distinguishing 

 lighthouses from one another by numbers, 



It is adapted in the Exhibition to a dioptric light exhibited by 

 Messrs. Chance. Major- General Babbage. 



The system of occulting lights for distinguishing lighthouses from one 

 another by numbers was invented by the late Charles JBabbage about the 

 year 1850. It was published in " The Exposition of 1851," and was favourably 

 reported on by the Lighthouse Board of the United States in January 1 852; 

 It was subsequently published in the "Times" of Wednesday, July llth, 



