IMl'KOVI.MKNT.S IN COAST SIGNALS. 



.391 



vertically, as shown, on !i system sufjgostccl by the lato Mv. Nicholas 

 Douglass, C.E., for the Haiinis Lighthouse near the island of Guern- 

 sey. From experiments which have been made upon blocks of hard 

 granite put together in this manner with Portland cement, it has been 

 found that the work is so homogeneous as to be as nearly as possible 

 equal to solid granite. Tiie system also affords great protection to both 

 horizontal and vertical joints against the wash of the sea and general 

 sa."ety to the work when lirst set. thus greatly faoilitiiting the execution 

 of such an exposed sea structure. Each stone of the foundation courses 

 is sunk to a depth of not less than one foot below the surrounding sur- 

 face of tlic rock, and is further secured to the rock bj two yellow metal 

 bolts. The tower contains nine rooms, which are fitted up for the 

 acconmiodation of the light- keepers and the stores necessary for the service 

 of the light. The rooms arc rendered as far as possible fire-proof, the 

 floors being of granite, the stairs and partitions of iron, and the external 

 doors, windows, and shutters of gun-metal. The two oil i-<inms arc 

 capable of storing 4,o00 gallons of oil, and the water tanks have a capacitv 

 of 4,700 gallons. The lantern is of the (cylindrical helically framed type 

 now generally adopted by the Trinity House. The distinctive cliariicter 

 of the light adopted for this station, to replace the old fixed light, i.s 

 white, double-Hashing at half-minute periods, showing two successive 

 flashes, each of about 31 seconds' duration, divided by an eclipse of 

 about 3 seconds, the second flash being followed by an eclipse of about 

 •JO seconds. The optical ajiparatus consists of two superposed tiers of 

 leu.ses, twelve in each tier. The section of these lenses, which are of the 

 first-order dimensions, having a common focus at a distance of tV20 

 millimetres, Avas designed by f)r. .lohn Hopkiiison, C.K., F.Ji.S., in ISSO, 

 for the Anvil Point Lighthouse. Each leus subtends a horizontal ano-le 

 at its focus of 'AO degrees, and a vertical angle of 92 degrees. This 

 increased vertical angle, which is the largest yet adopted for coast 

 illumination, has been obtained by the adoption of heavy flint-glass for 

 the six highest and for the three lowest rings of each lens panel. The 

 relative efficiency of this section of leus and that of the old section is 88 

 to 70 nearly, and its power is only about 12 per cent, less than that 

 of the complete section of a Frcsnel first order apparatus, composed 

 of lenses combined with totally reflecting prisms above and below them. 

 The focal light is produced by two six-wick ' Douglass ' oil-burners, one 

 being placed in the common focus of each tier of lenses. With a clear 

 atmosphere, and the Plymouth Breakwater light, ten miles distant, 

 clearly visible, the lower burner only is worked, and at its minimum 

 intensity of about 4-50 candle tmits, giving an intensity of the flashes of 

 the ojttical apparatus of about o7,000 candles ; but, whenever the atmo- 

 sphere is so thick as to impair the visibility of the Breakwater light, the 

 full power of the two burners is put in action, with an aggregate intensity 

 in the fla^lies of the optical apparatus of about l.')i*,000 candle units. This 

 intensity is about twenty-three times greater than that of the fixed diop- 

 tric light latterly exhibited from Snu'aton's tower, and about o,280 

 times the intensity of the light originally exhibited in the same tower 

 from tallow candles. The Eddystone furnishes complete evidence of the 

 recent progress in lighthouse illumination, and of the great value of 

 perfect optical apparatus for the utilisation of the illuminant for the 

 benefit of tho mariner. The original chandelier light in Smeaton's light- 

 house was unaided by optical apparatus, the intensity of the aggregate 



