These principles of the wind acting upon the sails, and the water upon the rudder, are the foundation of the whole science of working a ship. In large vessels the sails are numerous, but they may all be reduced to three classes, viz., head sails, or those which are forward of the centre of gravity or rotation, having a tendency to send the ship's head off from the wind; after sails, or those abaft the centre of rotation, and which send the stern off and the head toward the wind; and lastly, centre sails, which act equally on each side the centre of rotation, and do not turn the ship off her course one way or the other. These classes of sails, if set aback, tend to stop the headway and send the ship astern, and also to turn her off her course in the same direction as when set full, but with more rapidity. The further a sail is from the centre of rotation, the greater is its tendency to send the ship off from the line of her keel. Accordingly, a jib is the strongest head sail, and a spanker the strongest after sail. The centre of rotation is not necessarily at the centre of the ship. On the contrary, as vessels are now built, it may not be much abaft that part of the deck to which the main tack is boarded. For the main breadth, or dead-flat, being there, the greatest cavity will also be there, and of course the principal weight of the cargo should centre there, as being the strongest part. Therefore the centre of rotation will greatly depend upon proper stowage. If the ship is much by the stern, the centre of rotation will be carried aft, and if by the head, it will be carried forward. The cause of this is, that when loaded down by the stern, her after sails have but little effect to move her stern against the water, and a very slight action upon the forward sails will send her head off to leeward, as she is there light and high in the air. Accordingly, to keep her in a straight line, the press of sail is required to be further aft, or, in other words, the centre of rotation is further aft. If a ship is loaded down by the head, the opposite results follow, and more head and less after sail is necessary.