Some years since, Captain Morton patented what he called "the patent slip," for the purpose of hauling vessels on shore in positions and under circumstances wherein it was requisite to examine and repair their bottoms, and wherein it was either impossible or inex pedient to take the vessels into graving-docks. This slip consisted of the usual inclined way passing down under the high-water line. Upon this way, a platform on rollers, with proper stops, ratchets, and brakes, bearing also iron cradles adapted to receive the ship's aides, was made to work, by means of a steam-engine, in such a manner as to allow the platform to pass under the ship at high-water. The ship subsided into tho cradles with the falling tide ; and being then carefully wedged into them, it was hauled up upon dry land to be treated as required. Slips of this description have been found to answer remarkably well for vessels of less than 400 or 500 tons burden ; beyond those dimensions, the weight of the hulls becomes too great to be economically dealt with in this manner, and it becomes necessary either to resort to the use of the hydrostatic or floating docks, or to take the vessel into a graving-dock, if it should require repairs. Morton's slips are to be
found at most of the second-rate ports on our coast, and they are of extreme utility for our coasting.trade.
The most important consideration with respect to tho construction of building-slips is, that the floor should remain rigid under any possible load brought upon it ; for any inequality in the rate of settlement would not only increase the difficulty of launching, but it might very possibly produce a deformation of the lines of the vessel. As ship building almost necessarily takes place on the seashore, or on the banks of riven; flowing over alluvial deposits, there are very few occasions in which the natural foundations are sufficiently resisting; and it therefore becomes necessary to resort to the use of artificial foundations of some of the descriptions already mentioned. It is under the line of the keel that the greatest strain occurs, and that portion of the foundations must be treated with the greatest attention.