Floating docks themselves are in occasional need of repair, and when it was found that they could be constructed of a greater size than any then existing dry dock, it being customary to dry dock them for repair, the necessity arose of devising a means whereby the repairs could be made without taking the floating dock out of the water. Sometimes a dock can be tilted endways or sideways as occasion requires, for a portion of its under-water surface to be exposed, but there is obviously a limit to this operation and to the effectiveness with which work under these conditions can be carried out. This difficulty was met by constructing docks on the sectional principle, whereby any two sections of a floating dock constructed in three sections can lift the other one; while with off-shore docks, which are usually built in two sections, either can lift the other. An attempt to careen the old =[U]=-shaped Bermuda dock nearly capsized her altogether. One of the earliest--if indeed not the earliest--of self-docking double-sided docks is that associated with the name of Mr. Rennie, and now generally known as the Rennie type, or, in an attempt made at uniform classification of self-docking docks by Messrs. Clark and Standfield, who probably have had greater experience of floating-dock designing than any other firm in the world, the “sectional pontoon” dock. This is an extremely simple form of dock, consisting of a series of similar pontoons connected together into a whole by the walls or side girders, which run along each side on top of the pontoon, to which they are attached by bolts. In self-docking, any particular pontoon can be unbolted from underneath the walls, allowed to sink slightly, and then be drawn out sideways, turned half round, and lifted on the rest of the dock. The type is also very suitable for erection abroad, for the pontoons can be built and launched separately, and, being but light structures, require no expensive launching slips, whilst the side walls can be erected on top of the pontoons after they are afloat.[100] [100] “Modern Floating Docks,” by Lyonel Edwin Clark, M.I.N.A.