266 Hydrostatics.

in the VeJJ'el: For every Column of Par-ticles G H, which presses downwards on

the

Side A B be equal to twenty Pounds ; if a be the Cen-tre of Pressure, and a String abc were fix’d thereto go-ing over the Puilv b, and sustaining a Weight £ of 'twenty Pounds, then would the Side AB be kept inEquilibria by those two equal and opposite Powers.

18. Now the Centre of Pressure is the fame as theCentre es Percussion^ as is evident from hence, that in

p- - the Line AK the percussive Force of every Point G,

H, I, K, is as the Velocity, that is, as the DistanceAG, AH, Al, AK, from the Point A consideredas the Point of Suspension. But those Distances areas the Altitudes GB, HC, ID, K E, which are as thePressures on the foresaid Points: Therefore since thePressures, and percussive Forces are always alike, theCentre of one must be the same with that of the other,but the Centre of Percussion of the Line AB is twoThirds of its Length distant from the Point of Suspen-sion A, therefore also the Distance of the Centre ofPressure a is y of the Side AB below the Surface of theWater. See Annot. XXIX.

Scholium.

19. The Reason why a greater Force of Pressure is .allow’d to an oblique Plane or Bottom of a Vesstd thanwhat is equal to the Gravity of the Fluid, is this, thatevery Point in the Line A K is prefs’d with a perpendi-cular and lateral Force at the same Time, and by bothobliquely ; and of those two oblique Forces, or Pressures,one direct Force is compounded, which is greater thaneither, and acts in a Direction perpendicular to thePlane. But since both the oblique Pressures are always asthe Depth of the Fluid, the direct compound Pressurewill be in the fame Ratio likewise. Hence as there is

a greater Number of Points in the Line AK than in theLine A E, and each is press’d perpendicularly with aForce proportional to the Depth, ’tis necessary that weestimate the whole Force by the Area of the TriangleA e K, and not of A£K, as we must have done ifFluids press’d like Solids by their Gravity only.