182 THE SEA. 



CHAPTER XII. 



THE LIGHTHOUSE (concluded}. 



Lighthouses on Sand Literallj- screwed down The Light on Maplin Sands That of Port Fleet wood Iron Lighthouses 



The Lanterns themselves Eddystone long Illuminated with Tallow Candles Coal Fires Revolution caused by the 

 invention of the Argand Burner Improvements in Reflectors The Electric Light at Sea Flashing and Revoh ing- 

 Lights -Coloured Lights Their Advantages and Disadvantages Lanterns obscured by Moths, Bees, and Birds. 



THE difficulties involved in constructing- a lighthouse on solid rock have been shown,, 

 and it was at one time thought absolutely impossible to erect with any prospect of per- 

 manent duration one upon storm-exposed sands. Nous avons change tout cela. It is no- 

 longer necessary to place floating lights in places of great danger,, although for other 

 reasons they are constantly used. One of the greatest modern triumphs of engineering is 

 Mitchell's screw-mooring apparatus. To describe it fully would necessitate several pages of 

 technical matter. Suffice it to say that enormous cast-iron screws, having hollow cylin- 

 drical centres, through which wrought-iron spindles pass, are literally screwed down into- 

 the sand, or its substratum of other soil. One of the earliest experiments was made 

 on the v verge of the Maplin Sand, at the mouth of the Thames. Nine of the mooring-screws 

 were inserted into the sand 21| feet, one in the centre, the rest forming an octagon 42 

 feet in circumference, having standards or posts which stood 5 feet above the surface of 

 the sand. A raft of timber was floated over the spot, and a capstan in its centre drove 

 the screws to the required depth. This raft was afterwards sunk, by covering it with 

 200 tons of rough stone. Two years were allowed to elapse, at the termination of which 

 time the whole mass was found firmly embedded, and then a lighthouse, raised on a 

 strong open framework, was erected over this sub-structure. During these long pre- 

 parations a very similar structure was commenced and finished at Port Fleetwood, on the 

 River Wyre, near Lancaster. 



The preparatory steps were similar to those already described. The foundation of the 

 lighthouse was formed of seven screw-piles, six of them occuping the angles of a hexagon 

 46 feet in diameter, the seventh being in the centre. From each screw proceeds a pile 15 

 feet in length, having at the upper end another screw for securing a wooden column. 

 These columns are of Baltic timber, the one in the centre being 56 feet, the others 46 feet 

 in length, firmly secured with iron hoops and coated with pitch. The platform, upon 

 which the house stands, is 27 feet in diameter, the house itself being 20 feet in diameter 

 and 9 feet high. From the summit of the house rises a twelve-sided lantern, 10 feet in 

 diameter and 8 feet hiVh. Altogether the li<rht is elevated about 46 feet above low- 



o o o 



water level, and ranges over an horizon of eight miles. The light is of the dioptric kind 

 bright, steady, and uniform, and when the weather is too foggy to allow it to be seen, a 

 bell is tolled by machinery, to give the needful warning. 



At the period when screw-pile lighthouses were being thus successfully erected, other 

 and most valuable suggestions were being made for the building of bronze and cast-iron 

 lighthouses. The great advantage of iron over stone and other materials in those portions 

 of the building not actually in contact with sea-water soon became apparent. Upon a, 



