Page:The American Cyclopædia (1879) Volume X.djvu/467

 LIGHTHOUSE 461 of the top course 16 ft., and the whole height is 138 ft. The tower for a height of 26 ft. is solid. Immediately above the solid part the walls are 9 ft. thick, and they gradually dimin- ish from this thickness to 2 ft. The material is granite, and the tower is surmounted by a bronze lantern in which is placed a Fresnel lens of the first order, showing a revolving light. The work was commenced in 1839, and the light was first shown in February, 1844. An account of the construction of the work has been published by Mr. Stevenson, which is valuable not only for the description of this particular work, but because it contains a dis- sertation on the Fresnel system of lighthouse illumination, and a succinct history of light- houses. Figs. 8, 9, and 10 show the compara- tive sizes and shapes of the Eddystone, Bell Eock, and Skerryvore lighthouses. The high and low waters of ordinary tides are indicated on the figures, and the letter G- gives the posi- tion of the centre of gravity of each tower. A cast-iron lighthouse for the Great Isaac's rocks, near Bermuda, was completed in 1856, by order of the English admiralty. The tower is 120 ft. high from the base to the plane on which the lantern rests, and 150 ft. to the top of the lantern. At. the base the tower is 25 ft. in diameter, and at the top 14 ft. One of the great peculiarities of the construction is that the 155 large cast-iron plates of which it is composed are not placed horizontally round the tower, as heretofore in erections of a similar kind, but in what is technically called " break- joints ;" *'. 0., the plates, so to speak, are dove- tailed and wedged the one into the other, in such a manner as to form a perfect column, and equal in strength in all its parts. Other noted lighthouses have been erected upon the coasts of Europe, but those described are all whose constructions have been given in detail and published to the world, and are types of all others. Some on the coasts of France are as bold in their execution and as difficult in their construction as any noticed above. Many of the lighthouses in the United States are unsurpassed by any in the world, and are of exceedingly difficult construction. The most noted is that of Minot's Ledge off the coast of Massachusetts. This rock is the outer one of a ledge lying off the town of Oohasset. It is situated about 8 m. E. S. E. of Boston light, and is a projecting point very dangerous to vessels coming into Boston from seaward. Should these vessels have a N. E. wind, and by any chance miss the entrance to the harbor, they would be almost certainly cast away on these rocks were there' no signal placed there to warn them off. It is about 1 m. from the nearest land, and at low water the highest part of the rock (a circle about 25 ft. in diameter) is bare. The rise of spring tides is not far from 12 ft., so that no part of the rock is ever uncovered more than a few minutes. The difficulties of erecting a lighthouse on this rock were very great. Attention was drawn to the dangers of this point many years ago, and in 1847 an appropriation was made by congress for the construction of a lighthouse on the rock. It was determined to erect an iron-pile struc- ture, at the top of which was to be the keep- ers' dwelling, and this was to be surmounted by the lantern enclosing the illuminating ap- paratus. The plan of the work was a regular octagon, each side of which at the base was 9. ft., the diameter of the circumscribing circle being 25 ft. Iron piles 10 in. in diameter where they leave the rock were inserted 5 ft. into it, at each angle of the octagon and at its centre. These were firmly braced and tied to- gether by wronght-iron braces. At a height of 55 ft. above the highest point of the rock the heads of the piles were firmly secured to a heavy casting. Above this casting the floor of the dwelling was placed. The structure was finished in the autumn of 1849, and stood until April, 1851, when it was carried away by one of the most terrific storms that have ever occurred on the Atlantic coast. All of the iron piles were twisted off at short dis- tances above their feet. One cause of the de- struction of this lighthouse is supposed to have been a hawser which was fastened to the top of the structure at one end, the other being anchored in the sea. The waves after leaving the lighthouse would strike the hawser, and the effect of the blow was transmitted to the pyramid with very great leverage, causing a tendency to oscillate. Another cause was the ice which froze to the piles, and thus increased the extent of surface exposed to the action of the sea, In 1852 congress appropriated money for rebuilding the lighthouse, and a design was originated by the lighthouse board and ap- proved by the secretary of the treasury early in 1855. It is a granite tower in the shape of the frustum of a cone. The base is 30 ft. in diameter, and the whole height of the stone- work is 88 ft. The lower 40 ft. are solid. The remainder of the tower is made up of keepers' apartments, store rooms, and the par- apet, which encloses the pedestal of the lens apparatus. The stones of the courses are dovetailed in the securest manner, and the courses are fastened to each other by galvan- ized wrought-iron dowels, 3 in. in diameter. The work was commenced early in the season of 1855, and an idea of the difficulties to be overcome may be formed from the fact that although advantage was taken of every mo- ment in which it was possible to work upon the rock, it was not until the last part of the season of 1857 that any stones were laid, the whole of the intervening time having been taken up in levelling the foundation bed. In the season of 1857 four stones were laid, in 1858 six entire courses were laid, and in 1859 the whole of the solid portion of the structure and half of the remainder, making a total height of 60 ft., were placed. The lighthouse was finished and lighted at the end of 1860. The early history of lighthouses in the United