Lightships and Lighthouses

by Frederick Arthur Ambrose Talbot

When the lighthouse was projected, it was realized that it would have to be of massive proportions and provided with adequate measures to protect it from the assault and battering of the ice. The task was undertaken by General O. M. Poe, who was engineer-in-chief to General Sherman on his historic march to the sea. This engineer decided to take the Minot’s Ledge monolithic structure as his model, seeing that the latter had withstood the savage onslaughts of the Atlantic. Fortunately, the foundations were of an excellent character, the reef being formed of hard limestone.

By courtesy of Lieut.-Col. W. P. Anderson.

BUILDING THE BARRE À BOULARD LIGHT IN THE RIVER ST. LAWRENCE.

Owing to the severity of the ice piling in this waterway, the structures have to be provided with massive foundations.

The engineer selected as the site for the tower a point where the ridge is submerged by 11 feet of water. Seeing that the base was to be laid under water, obviously it seemed to be an operation for divers; but General Poe prepared a superior means of getting the subaqueous foundations laid. He built a cofferdam around the site, and, as the work would have to be protected from the winter ice, he built another cofferdam, entirely for protective purposes, outside the former. The nearest point on the mainland where he could establish a depot was Scammon’s Harbour, some sixteen miles away, and here everything in connection with the work was prepared and shipped to the site ready for placing in position.

The protective work comprised a wooden pier, built up of timbers 12 inches square, 24 feet in height. This structure was divided into a series of vertical compartments on all four sides, leaving a clear internal space 48 feet square. The outer compartments or pockets were filled with stone, to secure solidity and stability. Landing facilities were provided on this pier, together with quarters for the men engaged in the construction work.

In the inner space, containing 48 square feet of still water, the cofferdam, in which the subaqueous work was to be carried out, was lowered. This structure was cylindrical in form. It was built up of staves, banded with heavy hoops of iron, so that in reality it resembled a huge barrel 36 feet across. It was fashioned at the site, being built while suspended directly over the spot on which it was to be lowered. When the tub was finished, loosely twisted oakum, 1½ inches thick, was nailed all round the lower edge, while a flap of heavy canvas was secured to the outside bottom rim in such a way as to leave 36 inches dangling free. The exact circular shape of the cofferdam was insured by liberal cross-bracing from a central vertical post, which constituted the axis of the barrel, corresponding to the vertical axis of the tower. While this work was in progress, the face of the rock was cleared of loose boulders, and then the cofferdam was lowered bodily with extreme care, so that it descended with unerring accuracy perpendicularly into the water, to come to rest over the desired spot. As the surface of the reef was very uneven, the cofferdam stopped when it reached the highest projection under its edge. Then each stave of the barrel was driven downwards until it came to rest upon the sea-bed, and, as the oakum rope was forced down likewise, this served to act as caulking. The outer flap of canvas, when the cofferdam was driven right home, spread out on all sides, and lay upon the surface of the reef.

Pumps capable of discharging 5,000 gallons per minute then were set to work, removing the water from within the cofferdam. The oakum rope seal prevented the water regaining the internal space under the bottom edge of the tub, while the canvas assisted in securing absolute water-tightness, because the outer water-pressure forced it into all the nooks and crevices.

By these means the workmen were given an absolutely dry space in which to carry out their erecting work. The face of the reef was cleaned and levelled off, and the first layer of stones was laid. These were first fitted temporarily upon a false platform on shore, so that when they reached the site they could be set at once without finicking. The bottom layer is 32 feet in diameter, and the tower is solid to a height of 34 feet above the rock. The stones are each 2 feet in thickness, and are secured to one another on all sides with wrought-iron bolts, 24 inches long by 2½ inches in diameter; while the tower is anchored to the rock by cement and bolts 3 feet long, driven through the bottom course into the real rock beneath, entering the latter to a depth of 21 inches. Liquid cement was driven into the holes so as to fill up all the remaining interstices, and this now has become as hard as the stone itself.