[175] Between the screw and the rudder was what was termed the after stern-post, which had no duty to perform except to steady the heel of the ship, into which the rudder was stepped.
[176] Before the ship was finished, Mr. Brunel had her stability and rolling carefully investigated. Calculations were made to ascertain the position of her centre of gravity and the other necessary elements for determining her stability and period of rolling.
He also had a model made, with arrangements for altering the levels of weights placed inside. By this means the results of the calculations were verified. It was determined that the ship would make a single roll from one side to the other in about six seconds. While she was in the Thames a steamer struck the hulk alongside and gave the great ship a slight impulse. Mr. Brunel, who was on board, took advantage of the opportunity to observe the period of the roll she made, and was pleased at finding it agree with the calculated period. It is to the investigations initiated at that time by Mr. Brunel that are due the great steps since made in the knowledge of the laws which govern the rolling of ships. Had Mr. Brunel lived he would no doubt have taken the same pains to record the rolling of the ‘Great Eastern’ as he had in the case of the ‘Great Western’ when, in 1839, he sent an assistant to America and back, who took observations of the rolling and pitching of that vessel in several voyages. These observations were made by a simple angle-measuring instrument, adjusted by the visible level line of the horizon, and not by the fallacious method of noting down the swings of a pendulum.
The ‘Great Eastern’ remains almost perfectly steady in ordinary rough seas. When the seas become very long, so that their period is nearly the same as that of the ship, she rolls; though the number of degrees on either side of the perpendicular is not large. By stowing the weights of cargo high in the ship, the tendency to roll has been much diminished, and when engaged in cable laying, with the enormous weights in the cable tanks all placed above the lower deck, she is remarkably steady.
[177] History of the Atlantic Telegraph: New York, 1866.
[178] It may be desirable to give a short explanation of these works, which consisted principally of a sluice, a trunk, and a drag-boat.
The Sluice was made in the abutment of Prince’s Street Bridge, and was intended to create a scour after the water had been let off from either side of the float, the opening at the bridge being closed by a caisson which had long been in use when (as was formerly the case) the upper part of the float was scoured through Bathurst Basin.
The Trunk, near the entrance from Cumberland Basin, is a wooden culvert between the floating harbour and the river. As much of the mud as could be dragged there was deposited at the entrance of the trunk, and, when the tide was low in the new channel of the Avon, sluices were opened, and the water rushed through from the floating harbour, carrying the mud with it.
The Drag-Boat was fitted with a steam-engine which worked a large windlass with three compartments, round two of which chains were passed and fixed to posts on the quays, and the boat was dragged backwards and forwards. The third compartment of the windlass worked a chain which elevated or depressed a scraper, attached to a long pole at the stern, and secured from swerving by a chain-bridle which passed under the boat. The scraper stirred up the mud, and deposited the more solid parts at the entrance of the trunk.
Mr. Brunel also desired that the float boards of the Neetham Dam should be put into proper working order; and that they should be altered so that, in times of land floods, the whole or a considerable portion of the excess of water should be retained, and passed through the feeder; and that even the water of spring tides should be allowed to pass the dam, and then be stopped back for the same purpose.