Page:Transactions and Proceedings of the New Zealand Institute - Volume 1 (2nd ed.).djvu/488

454 Mr. George next gave some quotations from the "Encyclopædia Britannica," speaking in strong terms of the many advantages which slips possess over dry docks, particularly as to cost, which they quote as one to twenty; he explained the method of using and working the slip, and quoted some examples as to the favour with which slips are now being looked upon. Eor instance, a slip for raising vessels of 3,000 tons register was supplied by Messrs. Morton to the Egyptian Government; and Messrs. Inglis, of Glasgow, in 1867, erected a slip 800 feet in length for raising vessels of 3,000 tons, dead weight.

There did not appear to be on record any instance in which a ship has sustained permanent injury, when being placed on a slip, or in being launched. In the case of the first vessel placed on the Melbourne slip, she was satisfactorily raised out of the water, but, from the subsidence of the ways, would not run off again; the vessel was not permanently injured. The same difficulty as in Melbourne occurred in launching the "Great Eastern," in 1857, and with the iron-clad "Northumberland," of 6,650 tons register, and weighing 8,000 tons, at the Millwall Ironworks. The subsidence of the ways in the two last examples is not much to be surprised at, when we remember that the foundation of the ways consisted of Thames mud.

On the other hand, graving docks also are not free from liability to accident. At Marseilles, the "Imperatrice," a steam ship of upwards of 2,000 tons register, fell bodily a height of three feet, from the giving way of the struts, after the water had been pumped out of the dock, and everything moveable in the vessel was broken.

The principal objection urged against slips is, that in launching a vessel she would be liable, as the phrase goes, to " break her back," from the fact of her after part being afloat, and lifted by the action of the water, while her fore part was fixed in the carriage. This the author endeavoured to dispel by entering into a consideration of the force of waves during storms, and argued that a ship that could be so strained in being launched from a slip as to be at all damaged, would not be in a fit state to resist the action of the sea during a storm, and therefore would be much better in port.

Mr. George concluded by remarking, that extremes are dangerous in all things, and that he was not then prepared to assert the superiority of slips over docks, or docks over slips, but to show that those who are prepared to do so ought also to be prepared to support their assertions, either by citing some high authority, or by adducing facts in support of their assertions. Theory, practice, and science must all naturally be brought to bear on such a subject; docks have been subject to all three. Theory and science have been applied to the question of the value of slips, but more practice is