248 LECTURE XXVII. 



The counterpressure of the water, occasioned by the escape of a stream 

 from a moveable reservoir, was applied by Parent* to the purpose of turn- 

 ing a millstone, and various other authors have described machines of a 

 similar nature : they may be constructed with little or no wheel work, and it 

 does not appear to be necessary that much of the force of the water sliould be 

 lost in their operation ; but they have never been practically employed 

 with success, nor have they perhaps ever had a fair trial. 



The art of seamanship depends almost entirely on the management of 

 the forces and resistances of air and water, and if the laws of hydraulic 

 pressure, with respect to oblique and curved surfaces, were more completely 

 ascertained, we might calculate not only what the motions of a ship would 

 be under any imaginable circumstances, but we might also determine pre- 

 cisely what would be the best possible form of a ship, and what the best 

 arrangement of her rigging. 



When a ship is sailing immediately before the wind, little or no art is 

 required in setting her sails, and her velocity is only limited by that of the 

 wind and 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 con- 

 siderably modified. 



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 confined to its proper place : and this might 

 be done more effectually by changing 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 resistance 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 pro- 

 portions of the resistances opposed to the ship's motion in different direc- 

 tions, we may calculate from these resistances the magnitude of the angular 

 deviation or lee way : but hitherto such calculations have generally indi- 

 cated 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 confining 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 principal force of the 

 wind is applied to the anterior part of the ship, her head would be naturally 



* Hist, et Mem. de Paris, 1704. See Euler, Hist, et Mem. de Berlin, 1750, 

 1751, 1752. Waring, American Transactions, iii. 185. 



