With the growing size of vessels, greater docking facilities became necessary, and, as the commerce of the world increased and ports were developed, demands arose for docking accommodation which could not always be met, owing in some cases to financial difficulties, and in others to the engineering difficulties connected with the localities. As a solution of the problem, the floating dock, as it is known to-day, was invented. In spite of the opposition with which it was greeted, the new contrivance held its own, and its merits became generally recognised. The difficulties and the cost of constructing dry docks are very great, and the time taken in the work may run into years; one dock, indeed, is stated to have taken fifteen years to complete. As an instance of rapidity of floating-dock construction, the Vulcan Company of Stettin required a dock 510 feet long and of 11,000 tons lifting power at short notice. The complete dock with all machinery and fittings was launched within seven and a half months, and within eight months and thirteen days of the inception of the project, the dock, after being towed across the North Sea and moored in place at its site, was sunk ready to receive its first ship. The Havana dock was delivered at Havana within eleven months after the signing of the contract for its construction; the actual time expended on it, dating from the day the first plate was laid until the complete dock was launched, was six months and a day. Both these docks are of over 10,000 tons lifting power. How long would it have taken to excavate and build graving docks capable of receiving vessels of the size that these docks can accommodate? No dry dock can take a vessel larger than itself, and in reckoning the dimensions of a dock for receiving purposes it must be remembered that its cill is a fixture, that the width of the entrance at the cill must not be made greater than the strength of the structure will permit, and that though a dock may in other respects be able to receive a vessel it cannot do so if that vessel through any mishap should draw as much water as that at depth of cill, or if in heeling over, its bilges should be wider than the width of the dock entrance. None of these drawbacks apply to the floating dock. These immense modern structures of steel and iron can receive vessels longer than themselves, and in the case of the off-shore docks, can receive vessels wider than themselves.