160 MR. ALFRED FRYER : SUGGESTIONS 



cular room eighteen feet in diameter and eleven feet high, 

 and below it is the store-room fourteen feet wide and 

 seven feet in height. 



The structure weighs fifty-five tons, and when sunk to 

 the required depth displaces five thousand one hundred 

 and thirty feet of water or one hundred and forty-eight 

 tons. To counteract the buoyancy eighty-seven tons of 

 ballast are required, thirty-one of which are suspended 

 from the bottom as a sphere of cast iron seven feet in 

 diameter, while fifty tons of pig iron cover the floor of the 

 store-room ; the remaining weight of six tons is in stores. 



The upper light, eighty feet above the water, can be 

 seen at twelve miles distance; and the lower light (the 

 object of which will be explained afterwards) being thirty 

 feet above the water, can be seen at about eight miles 

 distance. The centre of gravity is thirty-three feet below 

 the surface of the water, and the centre of buoyancy 

 twenty-six feet ; thus the vessel is stable, and exerts con- 

 siderable power to regain the perpendicular if removed 

 from that position. The large disc mentioned above is 

 intended to steady the vessel and to reduce oscillation, as 

 no movement of a vibratory character can take place with- 

 out the flange displacing a proportionate quantity of water. 

 The inertia of a mass of one hundred and fifty tons, and 

 the time and power required for the disc to displace large 

 volumes of water, cause the vessel to be slow to obey the 

 proportionately slight impulse to rise and fall with the 

 waves. 



The surface on which the waves can beat is very small, 

 and is in fact less than the aggregate surface of the legs 

 of a screw-pile lighthouse. The wind also can exert but 

 little force upon so narrow a tube, which appears scarcely 

 thicker than a large ship^s mast, and like it presents a 

 taper form and convex surface. The structure is moored 

 by three anchors, the cables from which are attached to 



