X. LIGHTHOUSES, ETC. 533 



1855, and elsewhere. It is capable of being introduced into existing light- 

 houses without disturbing the existing lenses, c., and can be adapted to any 

 single lighthouse without interfering with the adjacent ones. The number 

 here shown is 587, thus : 



The light is visible during a long interval; five Decollations are made : 



The light is then visible during a short interval ; eight occupations are 

 made : 



The light is then visible during a short interval ; seven occupations are 

 made : 



Ihe light is then visible again during the long interval as before, and the 

 same series is repeated continually from sunset to sunrise : thus the seaman 

 by counting the cccultations becomes at once aware of the number of the 

 lighthouse, and knows from his chart his exact position. 



The lighthouses along the coast would, of course, not be numbered consecu- 

 tively ; in fact, no two adjacent lighthouses could have numbers containing 

 the same digit : for example, the lighthouses on either side of No. 587 might 

 be 839, 614, 587, 293, 418, so that a mistake could not occur in identifying 

 any one of them from the others. 



2182. Model of a Lighthouse now building upon the Little 

 Basses Hock, part of a reef about 7 miles S.S.E. of the coast 

 of Ceylon. Trinity House, London. 



The tower is of Scotch granite (Dalbeattie), each stone of which was 

 dressed, fitted, and marked in this country, freighted to Galle, and thence 

 canied to the rock and fixed in its place. The light is intended to be of the 

 first order, dioptric, on the group-flashing principle, showing two flashes in 

 quick succession every minute, at an elevation of 110 feet above high-water 

 spring tides. 



The rock is awash at low water, and is exposed to heavy seas during both 

 the N.E. and S.W. 'monsoons, and while the latter prevails is inaccessible 

 for work. The drawings show the methods of landing stone in a seaway by 

 steam-power. 



This lighthouse, as well as its fellow on the Great Basses just completed, ie 

 building from designs by James N. Douglass, M. Inst. C.E., under the super- 

 intendence of William Douglass, M. Inst. C.E., for the Corporation of Trinity 

 House, London, acting on behalf of the Home and Colonial authorities. 



Its cost is estimated at 73,000/., and completion is anticipated within five 

 years from date of commencement. 



2182a. Lantern and Apparatus intended for the Little 

 Basses Lighthouse, Ceylon. Trinity House-) London. 



The lantern is of the cylindrical type adopted by the Trinity House, its form 

 gives maximum strength, and secures greater optical accuracy than the earlier 

 methods of flat glazing. The gun-metal framing is inclined about 30 from 

 the perpendicular, and is helically curved throughout, thus reducing to a 

 minimum the obstruction offered to the light sent forth from the lenses. 



The optical apparatus, constructed upon the group-flashing principle, 

 designed by J. Hopkinson, B.A., D.Sc., at the glass works of Messrs. 

 Chance Brothers, is the first dioptric ins'trument of its kind adopted by the 

 British lighthouse authorities. It is 12-sided, and makes a completed revolu- 

 tion in six minutes, so that the panels being arranged in pairs, a double-flash 

 meets the eye of the observer once a minute. 



The lantern and apparatus prepared for this structure are exhibited in 

 working order in the grounds outside the Museum. 



