A Boat or Vessel of any Size (a 90 Gun Ship moving upon the same Principle as the smallest Cutter,) is acted upon by the Powers which are the Cause of her Motion as she swims in the Water, in the same Manner as a Lever of the first Kind, whose Center or Prop is between the Power and the Weight. To explain this, let us suppose a Boat, instead of Swimming in the Water, to be upon dry Land, and to have her Mast run quite through her, and fastened into the Ground, upon which she might be turned at Pleasure, as upon an Axle-Tree: In this Case, as her Mast is rather nearer her Head than her Stern, it would be more easy to turn her Head round by laying hold of her Stern, because there would be a Mechanical Advantage, by the greater Length from the Stern to the Mast, than from the Head to the Mast. And in whatever Direction the Stern of the Vessel is turned, her Head must move the contrary Way, and vice versâ. Now, the same will happen to a Vessel in the Water; if you push her Head in one Direction, her Stern will move in the other, and vice versâ. So that a Vessel under Sail with a Side-wind, may be called a Lever of the first Kind, both whose Extremities are kept in a Ballance by the Sails and Rudder; Forces which keep continually acting upon her. The Rudder may be considered as a Kind of Moderator, which is to interpose when the Sails which are before the Mast, or those which are behind the Mast, or abaft, overpower each other, and destroy that Ballance which a Vessel rightly trimmed very near preserves of herself. It must be observed, that the Sails before the Mast of a Vessel, and those behind it, act in contrary Directions. Those which are before the Mast turn her Head from the Wind, and those which are behind it turn her Head towards the Wind.