112 LIGHTHOUSE 



distance of 36-22 inches, and are lighted by a lamp of four concentric wicks, con- 

 Burning 570 gallons of oil per annum. Eecently (1874) mineral oil has been used 

 with much advantage and economy in the lighthouses of America. 



The following notices may be of interest : The Eddystone Lighthouse, 9 miles 

 from the Eame Head, on the coast of Cornwall, was erected of timber by Winstanley 

 in 1696-98, and was washed away in 1703. It was rebuilt by Eudyard in 1706, 

 and destroyed by fire in 1755. The present edifice was erected by Smeaton 1757-59. 

 Tallow candles were used in the first instance for the lights; but in 1807 argand 

 lamps, with paraboloidal reflectors of silvered copper were substituted. 



The Skerryvore Kocks, about 12 miles south-west of Tyree on the coast of Argyle- 

 shire, lying in the track of the shipping of Liverpool and of the Clyde had long been 

 regarded with dread by the mariners frequenting these seas. The extreme difficulty 

 of the position, exposed to the unbroken force of the Atlantic Ocean, had alone de- 

 terred the Commissioners of Northern Lights from the attempt to place a light upon 

 this dangerous spot; but in 1834 they caused the reef to be surveyed, and in 1838 

 Mr. Alan Stevenson, their engineer, inheriting his father's energy and scientific skill, 

 commenced his operations upon a site from which ' nothing could be seen for miles 

 around but white foaming breakers, and nothing could be heard but the howling of 

 the winds and the lashing of the waves.' His design was an adaptation of Smeaton's 

 tower of the Eddystone to the peculiar situation, a circumstance with which he had 

 to contend. He established a circular base 42 feet in diameter, rising in a solid 

 mass of gneiss or granite, but diminishing in diameter to the height of 26 feet, and 

 presenting an even concave surface all round to the action of the waves. Imme- 

 diately above this level the walls are 9'58 feet thick, diminishing in thickness as the 

 tower rises to its highest elevation, where the walls are reduced to 2 feet in thickness, 

 and the diameter to 16 feet. The tower is built of granite from the islands of 

 Tyree and Mull, and its height from the base is 138 feet 8 inches. In the in- 

 tervals left by the thickness of the walls are the stairs, a space for the necessary 

 supply of stores, and a not uncomfortable habitation for three attendants. The 

 rest of the establishment, stores, &c., are kept at the depot in the island of Tyree. 

 The light of the Skerryvore is revolving, and is produced by the revolution of 

 eight annular lenses around a central lamp, and belongs to the first order of dioptric 

 lights in the system of Fresnel, and may be seen from a vessel's deck at a distance 

 of 18 miles. 



Some of the lenticular arrangements must now claim attention. The appearance of 

 light called short eclipses has hitherto been obtained by the following arrangement : 

 An apparatus for a fixed light being provided, composed of a central cylinder and two 

 zones of catadioptric rings forming a cupola and lower part, a certain number of lenses 

 are arranged at equal distances from each other, placed upon an exterior moveable 

 frame making its revolution around the apparatus in a given period. These lenses, 

 composed of vertical prisms, are of the same altitude as the cylinder, and the radius 

 of their curves is in opposite directions to those of the cylinder, in such a manner 

 that at their passage they converge into a parallel pencil of light, all the divergent 

 rays emitted horizontally from the cylinder producing a brilliant effect, like that 



obtained by the use of annular lenses at the 

 1385 revolving lighthouses. Large lenses, or any 



large masses of glass, are liable to striae, 

 which by dispersing, occasion a loss of much 

 light. 



' In order to improve a solid lens formed of 

 one piece of glass whose section is A, m, p, B, 

 F, E, D, c, A (fig. 1384), Buffon proposed to cut 

 out all the glass left white in the figure, 

 namely, the portions between m p and n o, and 

 between o and the left-hand surface of E. 

 A lens thus constructed would be incomparably 

 superior to a solid one, but such a process we 

 conceive to be impracticable on a large scale, from the extreme difficulty of polishing 

 the surfaces A m, B p, c n, F o, and the left-hand surface of D E ; and even if it were 

 practical, the greatest imperfections of the glass might happen to occur in the parts 

 which are left. In order to remove these imperfections and to construct lenses of 

 any size,' says Sir David Erewster, 'I proposed in 1811 to build them up of separate 

 zones or rings, each of which rings was again to be composed of separate segments, 

 as shown in the front view of the lens in fig. 1385. This lens is composed of one 

 central lens A B c o, corresponding with its section j> B in fig. 1384 ; of a middle ring 

 c. F. i. i, corresponding to c D, E F, and consisting of 4 segments ; and another ring 

 N p E T, corresponding to A c, F B, and consisting of 8 segments. The preceding 



