OX THE REGULATION OF HYDRAULIC FORCES.

by the resistance of the water: but for sailing with a side wind, it becomesnecessary that the immediate force of the wind should be considerably mo-dified.

If we had a circular vessel or tub, with a single mast, and a sail perfectlyflat, and if the sail were placed in a direction deviating but little from that ofthe wind, the tub would begin to move in a direction nearly at right anglesto that of the wind, since the impulse of the wind acts almost entirely in adirection perpendicular to that of the sail: but the slightest inequality of thedimensions of the sail, or of the force of the wind, would immediately disturbthe position of the vessel; and in order to avoid this inconvenience, it wouldbe necessary to have a moveable body projecting into the water, so as to createa resistance by means of which the vessel might be steered, and the sail con-fined to its proper place : and this might be done more effectually by chang-ing the form of the vessel from round to oval; it would then also have theadvantage of moving much more easily through the water in the direction ofits length than a circular vessel of equal size, and of creating still more "re-sistance in a transverse direction, so that when urged by an oblique force, itwould move in some measure obliquely, but always much more nearly in thedirection of its length than of its breadth. The angular deviation from thetrack of the ship is called its lee way, and if we know the direction of thesails, and the actual proportions of the resistances opposed to the ship’smotion in different directions, we may calculate from these resistances themagnitude of the angular deviation or lee way: but hitherto such calcula-tions have generally indicated a lee way three or four times as great as thatwhich has been observed. The use of the keel is not only to assist in confin-ing the motion of the ship to its proper direction, but also to diminish thedisposition to vibrate from side to side, which would interfere with theeffect of the sails, and produce many other inconveniences. When the prin-cipal force of the wind is applied to the anterior part of the ship, her headwoidd be naturally turned from the wind if the rudder were not made to pro-ject from the stern in a contrary direction, and to present the surface of aninclined plane to the water which glides along the keel, so as to preserve theship, by means of the pressure which it receives, in any direction that may berequired for her manoeuvres. Commonly,however, although the sails may be soarranged that the principal force of the wind appears to be on the fore part of