A structure similar to this was proposed by Mr. Bobert Stevenson in 1800, for the Bell-rock lighthouse. It was intended to affix the foundation to the rocks, and that the iron shafts should support several stories ; whereas the. Maplin and Wyrc lights have but a single story. A similar struc ture is also now building on the rocks of Minot's Ledge off Boston, in the United States ; and another is constructing in London for the Bishop Rock off Scilly, designed by Mr. Walker. N r. Mitchell previously completed a lighthouse upon a similar foundation at the mouth of the Wyre river, in Morecambe hay, about 30 miles north of Liverpool. It was commenced in November, 1839, and lighted in June, 1840. The foundation is formed of seven screw-piles, six in a circle, and one in the centre, each pile being five inches in diameter, with a screw of three feet diameter. The screws were sunk 13 feet into the bank, which is composed of' exceedingly hard sand. On these screws is supported the lighthouse, consisting of one floor only, and the lantern above it.
Another plan has been carried into effect, at the Point of Air lighthouse, at the entrance of the river Dee, near Chester. This, which is similar in superstructure to the Maplin lighthouse, is by Messrs. Walker and Burgess, and consists of nine hollow iron cylinders, 3 feet 9 inches in diameter, sunk 12 feet into the sand by aid of an instrument known to well-sinkers as "the Miser," which extracts the sand contained in the cylinder. In these the bases of the piles are inserted. and then filled with concrete. But this, it must be observed, is erected above low water-mark.
While on this point of the subject we may notice an admirable plan for forming a tbundation for a bridge or pier of similar structure described by Mr. Charles Fox, of the firm of Fox, Henderson, and Company, in his evidence before the Committee on the Westminster Temporary Bridge bill in 1850.
This plan has been adopted with great success in the several bridges on different lines of railway, built by Messrs. Fox and Company, under the direction of Mr. Cubitt and other engineers ; and though not yet, as we believe, applied to the building of lighthouses, seems well adapted to that purpose.
We cannot better describe this plan than in Mr. Fox's own words, in the following extract from the printed evidence of the Committee above mentioned.
"Will you describe to the Committee the mode of con struction ?—Perhaps the simplest mode of describing it is to say, that instead of using the old-fashioned wooden coffer dam, which was always a temporary work, we make use of cylinders of iron, which are in themselves coffer-dams, and which remain permanently as a portion of the structure.
We adopt various modes of getting them down, but the more general one is this : we have a large receiver of wrought-iron, very much like a cylindrical high-pressure boiler, and from that receiver we exhaust the atmosphere, and when we get the cylinder put into its place, just care ffilly lowered down on to the bed of the river, surrounded by temporary frames of timber, so as to be sure that it shall be kept in a vertical position, we put a cap on to the top, having an elastic pipe from the top cap to the exhausted receiver, and we, at the proper time, open the communication between the two, and the pressure of the atmosphere on the surface of the water in the river produces such a rush to fill up the tube, as to get rid of any vacuous space, that it carries on a constant state of excavation under the bottom edge of the cylinder. from the pressure of the atmosphere on the top. The atmosphere takes care to push down the pile, aided b? its own weight, so as to take up any little space that II may have been excavated. When this mode was first spoken of, it was treated with a great deal of ridicule, and people natu rally said, Why, if the pressure of the atmosphere will push the pile down, when the pile is down it will not carry more than a weight equivalent to the pressure of the atmosphere ;' and a very practical man raised that objection ; not a N'ery scientific man, but a man of very great experience ; and 1 said to him, Now you are quite wrong, for the principle is, that it acts as a sort of excavating process ; it is quite trite that the pressure of the atmosphere on the top is useful, as it gets over any little friction on the sides of the tube so as to enable it to follow into the excavated space, and without that principle we could not push the cylinder down at all.' To prove this, we took a six-feet cylinder, and calculated what the pressure of the atmosphere upon that would be, and taking the whole pressure of the atmosphere, it amounted to about 30 tons. I had 30 tons of iron rails placed on the top of the cylinder, and the only result was, that it pushed it down about three-quarters of an inch into the gravel and brought it to a bearing, but it did no more.
" Was that upon a cylinder of six feet in diameter 1—Yes ; we then took off the 30 tons of iron-rails and put on the cap and opened the communication with the exhausted receiver, and the cylinder immediately descended into the solid gravel ti feet 6 inches by one impulse.