ON THE REGULATION OF HYDRAULIC FORCES. 3SS 



by the resistance of the water: but for sailing with a side wind, it becomes 

 necessary 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 perfectly 

 flat, and if the sail were placed in a direction deviating but little from that of 

 the wind, the tub would begin to move in a direction nearly at right angles 

 to that of the wind, since the impulse of the wind acts almost entirely in a 

 direction perpendicular to that of the sail : but the slightest inequality of the 

 dimensions of the sail, or of the force of the wind, would immediately disturb 

 the position of the vessel ; and in order to avoid this inconvenience, it would 

 be necessary to have a moveable body projecting into the water, so as to create 

 a 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 the 

 advantage of moving much more easily through the water in the direction of 

 its 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, it 

 would move in some measure obliquely, but always much more nearly in the 

 direction of its length than of its breadth. The angular deviation from the 

 track of the ship is called its lee way, and if we know the direction of the 

 sails, and the actual proportions of the resistances opposed to the ship's 

 motion in different directions, we may calculate from these resistances the 

 magnitude 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 that 

 which 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 the 

 disposition to vibrate from side to side, which would interfere with the 

 effect 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 head 

 would 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 an 

 inclined plane to the water which glides along the keel, so as to preserve the 

 ship, by means of the pressure which it receives, in any direction that may be 

 required for her manoeuvres. Commonly,however, although the sails may be so 

 arranged that the principal force of the wind appears to be on the fore part of 



