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Hydrostatics

mit the Length of the Sluice be 20 Feet,the Depth of Water 5 ; then will the Areaunder Water be 100 square Feet; whichmultiplied by 2 4 ? the Depth of the Centreof Gravity, gives 250 cubic Feet; whichagain multiplied by 62s, gives 15625/i.equal to 7 Tons nearly (LXIV).

Again : Since the Weight of Bodies is al-ways as the specific Gravity in equal Bulks ,it follows, that the Numbers in the Tabledo also express the Number of Averdupois

Ounces

(LXIV) 1. That the Area under Water multipliedby the Depth of the Centre of Gravity gives the Num-ber of solid Feet of Water pressing against the Sluice, isevident from Annot. LV. Art. 13, 14, 15, tAc. Andthis Product is to be multiplied by 62), because somany Pounds is the Weight of one solid or cubic Footof Water. See Annot. LXIl. Art. 7.

2. This Example gives the Numbers which I tookfrom a Pen that was made across a River; and thoughit may seem wonderful that all the Water in the Rivershould affect the Pen with no Force of Pressure, yetthis will be found consistent with the Laws of the Actionof Fluids, when that is understood which we have de-livered in Annot. LVI. For the Water of the Riverdoes no otherwise influence the Pen, than as it sustainsa Plane of fluid Particles to a certain Height in imme-diate Contact with the Pen. This single Plane of Par-ticles it is that gives all the Pressure on the Pen; andthey would do the fame, could they be otherwise sup-ported against it, if all the rest of the Water in theRiver were taken away. Thus the Waters of the Seaor Ocean lying against any Dam or Bank will press itwith no greater Force than the least Quantity of Wa-ter standing to the fame Height, over the fame Extentof the Dam.