In such huge and complicated structures as ships have grown to be, repairs, naturally, are frequent and vital. The ordinary wear to which the machinery is subjected necessitates constant adjustments and replacements. Improved mechanical apparatus sometimes is installed to take the place of less reliable or less economical apparatus. The action of sea water on the exposed metal and the collection below the water line of barnacles and other marine growths require periodic attention, while paint seems for ever necessary and, at least on warships, wet paint is omnipresent. Before the introduction of iron and steel, ships were comparatively small, and consequently it was a simpler job to haul them out of water or ground them at high tide in order that, when the tide had gone out, their underbodies could be examined and repaired. Sometimes, again, tackle made fast to their masts and led to anchors dropped well away from their sides or to points ashore made it possible for ships to be hauled over to one side or the other, bringing a large part of their underbodies above water, where their crews could make the necessary repairs, or scrape off most of the accumulation of marine growth. [Illustration: THE _OLYMPIC_ _A sister ship of the ill-fated_ Titanic, _and operated by the White Star Line._] Nowadays, however, when the very smallest of our ocean-going steamers is many times the size of Columbus’s largest ship, such methods avail little. Sometimes, still, in harbours where there is a large rise and fall of tide the smaller ships avail themselves of it for minor repairs, but for most modern ships such methods are impossible and dangerous. Yet even the greatest ships must from time to time be taken out of the water for repairs and for the inspection of the hulls, and for this purpose dry docks, or, as they are sometimes called, graving docks, came to be designed. Dry docks are long narrow basins, the dimensions of which are slightly larger than the largest ships they can accommodate. Nowadays they are usually built of reenforced concrete, although brick and stone are sometimes used, and formerly timber dry docks were not uncommon in the United States. The entrances to these basins are equipped with hinged gates, or a floating or sliding caisson. Dry docks in the United States ordinarily use the floating caisson. European dry docks commonly use the other two. These seal the mouths of the dry docks, preventing the entrance of water from the outside as powerful engines pump the water from the dock itself. The sides of dry docks are usually built in steps, so that at the top they are wider than at the bottom. The bottom is very nearly level, but there are careful arrangements made for draining all the water into pits from which it is pumped out.