Once more the crew becomes active. Girders are lowered into their places across the twenty-foot-wide hatches. Great planks cover the opening, and several huge tarpaulins are unrolled and spread above the planks, for cargoes must be guarded against salt water. These coverings are carefully put in place while the stokers raise the boiler pressure once more, and ere the last of the preparations is completed another voyage has been begun. * * * * * There are many other types of ships that busy themselves about the sea. One of these is the oil tanker, a ship built for but a single purpose. These are owned by the big oil companies whose products come from Mexico or the Dutch East Indies, or, originating in the United States, are sold to countries not so fortunate as to have oil wells of their own. [Illustration: AN OIL TANKER _These ships have come to the seas in very recent years. They are used only for the transportation of oil, and are owned largely by the great oil companies._] An oil tanker has an appearance more or less its own, although the great carriers of ore and grain on the Great Lakes are very similar. On these ships the engines and boilers are in the stern, and sometimes, too, the bridge is there, with the funnel rising from behind it, in a position which few sailors can accept as normal. Sometimes, again, the bridge and a small deck house are amidships. On these tankers the propelling machinery is in the stern in order that the cargo may be insulated to the greatest possible extent from the fires. Incidentally, too, it is the empty tanker that requires the most care, for just as an empty gasolene tin will explode while one filled to overflowing with gasolene will not, so the empty tanker, reeking with the gas left by the oil it carries, is more apt to explode. The turret steamer falls into almost any category. It is built in order to save money on certain port and canal dues and other taxes, and its appearance is perhaps the weirdest of that of any ship, save, perhaps, the antediluvian whalebacks once so common on the Great Lakes. Below the water line these turret steamers are much the same as other freighters, but from there up they are vastly different. Just above the water line their sides are turned in until they are almost a deck. These “decks” run forward nearly to the bow and aft almost to the stern. But the central portion of the ship from bow to stern is raised ten or a dozen feet above these strange side “decks,” which in reality are not decks at all, but only sections of the sides of these strange hulls. The turret ships have few, if any, advantages over more normal ships, their only purpose being to save what money they can in tolls that ships less strangely designed are forced to pay. The turret ship is only the naval architect’s way of making it possible for the ship’s owners to take advantage of certain technicalities in wording. They are few in number and are of minor importance.