LECTURE XXVII.

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of a different nature. The inclination of the axis of a windmill to the horizon isprincipally intended to allow room for the action of the wind at the lowerpart, where it would be weakened if the sails came too nearly in contact withthe building, as they must do if they were perfectly upright. When it isnecessary to stop the motion of a windmill, a break is applied to the surfaceof a large wheel, so that its friction operates with a considerable mechanicaladvantage. Water wheels with oblique floatboards are sometimes used withgood effect in China and in the south of Fiance : for title wheels, such float-boards have the advantage that they may be easily made to turn on a hingewith the stream, so as to impel the wheel in the same direction whether thetide be flowing or ebbing. (Plate XXII. Fig. 293 .)

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 firstcauses a pressure perpendicular to the surface of the kite, and this force,combined with the resistance of the string, produces a vertical result capableof counteracting the weight of the kite. (Plate XXII. Fig. 291.)

The counterpressure of the water, occasioned by the escape of a stream froma moveable reservoir, was applied by Parent to tlie 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 appearto be necessary that much of the force of the water should be lost in theiroperation; but they have never been practically employed with success, norhave they perhaps ever had a fair trial.

The art of seamanship depends almost entirely on the management of theforces 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 underany imaginable circumstances, but we might also determine precisely whatwould be the best possible form of a ship, and what the best arrangement ofher 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