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. The exterior of the tower is the frustum of a cone, and at 80 feet above the base is 18 feet in diameter. The total height of the masonry is 93 feet, and the focal plane is brought 97¼ feet above the rock, or 86¼ feet above the water-level. The tower is provided with five rooms, each 14 feet in diameter, while the entrance is 23 feet above the water. The undertaking was commenced in May, 1870, and the light was shown first in June, 1874. As work had been confined to the summer months, and a fortnight every spring was devoted to preparations, as well as an equal period in the autumn to making all fast to withstand the rigours of winter, the total working period was only some twenty months. [Illustration: _By courtesy of Lieut.-Col. W. P. Anderson._ COLCHESTER REEF LIGHTHOUSE, LAKE ERIE. An isolated station maintained by the Canadian Government. It is a fixed light, visible throughout a circle of 16 miles radius.]