74 ANNUAL OF SCIENTIFIC DISCOVERY. 



THE MINOT ROCK LIGHT-HOUSE IN BOSTON HARBOUR. 



FROM the report of Captain W. H. Swift, of the Topographical 

 Engineers, we derive the following account of the new light-house off 

 Cohasset, in Boston Harbour. The rock selected for the site of the 

 light-house is called the Outer Minot; at extreme low-water an area 

 of about 30 feet in diameter is exposed, the highest point being about 

 3i feet above low-water line. The rock is granite, with vertical scams 

 of trap rising through it. The form of the light-house frame is an 

 octagon, of \b feet diameter at base. The structure is formed of 

 8 heavy wrought-iron piles or shafts, placed at equal distances from 

 each other, with one also at the centre. These piles are forged in 

 two pieces each, and are connected together by gun-metal sockets, the 

 interior of which is bored, and the pile-ends are turned and secured 

 to the sockets by means of large steel keys passing through the piles 

 and the sockets. Above and below the joints or sockets, and connect- 

 ing the middle pile with each outer pile, there extends a series of 

 wrought-iron braces ; and the outer shafts are connected together by 

 similar braces, extending from one to the other, and thus the whole 

 structure is tied together. At each of the angular points in the octa- 

 gon, and at the centre, a hole of 12 inches in diameter and 5 feet in 

 depth, is drilled in the rock; the outer holes with the inclination 

 or batter given to the outer piles, and the middle hole vertical. The 

 piles are of unequal lengths, the least length in the lower series being 

 35? feet, and the greatest 38|. The piles in the upper series are of 

 the uniform length of 25 feet each ; the batter of the piles towards 

 the centre brings the heads of the upper ones within the periphery of 

 a circle of 14 feet diameter, and there, at an elevation of 60 feet above 

 the base of the middle pile, the pile-heads are secured to a heavy cast- 

 ing or cap, to the arms of which they are securely keyed and bolted. 

 The middle shaft is 8 inches in diameter at foot and 6 inches at top, 

 and the outer shaft 8 inches at foot and 4i at top, all being forged 10 

 inches in diameter at the point where they leave the surface of the 

 rock, and tapering uniformly down to 8 inches ii\ diameter in both di- 

 rections, within a distance of 5 feet. The lower braces, placed 19 

 feet above the rock, are 3i inches in diameter; the second series, 19 

 feet above the first, are 3 inches in diameter, and a third series, intro 

 duced 8^ feet below the cast-iron cap, to form the support of the floor 

 of the store-room, is made of 2i-inch-square iron. 



The outer piles being inclined towards the centre, and the piles and 

 the braces being inflexible, it is clear that, so long as the braces remain 

 in place, the pile cannot be withdrawn from the hole, for the whole 

 structure acts as an immense " lewis ;" either the braces must be rup- 

 tured, or the rock itself must yield, before a pile can be displaced. 

 Upon the pile-heads are cast-iron sockets, furnished with arms 3 feet 

 in length, pointing outwards. These sockets are keyed to the heads 

 of the piles, and are bolted to the arms of the cap or spider, flush with 

 its upper surface ; thus giving a diameter of 20 feet from out to out. 

 The object of the arms is to afford support for a gallery outside of 

 the keeper's house, which is secured directly to the cap by bolts or 



