324 LECTURE XXVII. 



of a different nature. The inclination of the axis of a windmill to the horizon is 

 principally intended to allow room for the action of the wind at the lower 

 part, where it would be weakened if the sails came too nearly in contact with 

 the building, as they must do if they were perfectly upright. When it is 

 necessary to stop the motion of a windmill, a break is applied to the surface 

 of a large wheel, so that its friction operates with a considerable mechanical 

 advantage. Water wheels with oblique floatboards are sometimes used with 

 good cifect in China and in the south of France : for tide wheels, such float- 

 boards have the advantage that they may be easily made to turn on a hinge 

 with the stream, so as to impel the wheel in the same direction whether the 

 tide be flowing or ebbing. (Plate XXII, Fig. '2.93.) 



A smoke jack is a windmill in miniature ; a kite affords a very familiar ex- 

 ample of the effect of the oblique impulse of the air, of which the action first 

 causes a pressure perpendicular to the surface of the kite, and this force, 

 combined with the resistance of the string, produces a vertical result capable 

 of counteracting the weight of the kite. (Plate XXII. Fig. 294.) 



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

 a moveable reservoir, was applied by Parent to the purpose of turning a mill- 

 stone, 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 should 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 ascertain- 

 ed, we might calculate not only what the motions of a ship would be under 

 any imaginable circumstances, but we might also determine precisely 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 

 m setting her sails, and her velocity is only limited by that of the wind, and 



