Page:Transactions and Proceedings of the New Zealand Institute - Volume 1 (2nd ed.).djvu/487

Rh the instant required for the difference between the longitude of the place east or west of 172° 30′ with the true time, telegraph time can be shown if desired.

4. "On the Merits of Patent Slips," by J. R. George, C.E.; read by Mr. Travers, in the absence of the author.

The author commenced by observing that as much apprehension appears to exist in reference to the safety of patent slips, for the purpose of raising vessels of large tonnage out of the water, he had been induced to offer a few observations on the mode of working such slips, in order to show that it is premature to assert the absolute superiority of graving or floating docks. There appeared to be great difficulty in dealing with the subject, from the fact that very scanty data exist, and there is no work of reference as yet published on the subject.

The terms "slip" and "wet dock" were formerly synonymous, and implied a narrow inlet of the same form as an ordinary graving dock, but with an inclined bottom, and having no flood gates; this form of slip was, consequently, only in use in situations having a large rise and fall of tide.

The Americans appear to have first invented slips, and worked them under the name of "Patent Submarine Railways." The first patent granted by the British Grovernment for slips was in 1832, to Mr. Morton. In the report of the Committee appointed to consider the renewal of that patent, they remarked that by means of a slip a vessel could be placed in a situation to be repaired at a cost of £3, which previously amounted to £170; this is the first strong authority as to their capacity and usefulness for raising vessels.

It appears, from competent authority, that a durable and substantial slip may be constructed, under favourable circumstances, at about one-tenth of the expense of a dry dock; and be laid down in situations where it is almost impossible, from the nature of the ground, or the want of a rise and fall of tide, to have a dock built. Some of the other advantages deserving of mention are: the air has a free circulation all around the ship, and there is better and longer light than within the walls of a dry dock; there is a considerable saving of expense in the carriage of the necessary materials; the vessel is exposed to no strain whatever; and she may be hauled up, repaired, and launched, within a few hours, no interruption taking place, as in docks, from the necessity of emptying and filling the dock with water.

Messrs. Morton add further, that ships of 2,000 tons register have frequently been drawn out of the water by means of their slip; and they estimate the relative cost of slips and docks as one to ten.