An Explication of the Second Plate.

Figure 1. Is a large Glass Vessel AD full of Water as high as EF. Within this is a lesser Glass Vessel PH, open at both Ends, but somewhat narrower at the Bottom. Through the middle of this goes a strong Wire MN, to which is fixed at the lower End a Plate of Lead GH, with wet Leather to its upper Surface, to be applied to the large lower Orifice of the lesser Glass IK, to keep out the Water from entring into the same any otherwise than by a slow Insinuation. This is to shew that a Plate of Lead, or other Metal, may be supported by Water, and not sink in it, where the Water is kept from pressing on its upper Surface, so long as its Depth under the Water is greater than its Specifick Gravity requires; and that by Consequence while Water is gradually admitted over it, it will not sink till the perpendicular Height of the Column of Air between EF and RS bears no greater Proportion to the Thickness of the metalline Plate (with what is annexed to it) than the Specifick Gravity of the Metal bears to Water.

Fig.2. Is a cylindrical Vial or Glass AD, with a small Cylinder of Wood below GH fixed to its Bottom, and made very smooth at Top; and another like Cylinder of Wood above GH, made equally smooth on the lower Side, that it may as exactly as possible fit the other; with a strong Pin I, fixed in its Axis. Upon these Two, when laid close, is pour'd Quicksilver, till it covers them both as far as EF. This is to shew, that there is no such thing as positive Levity; but that Wood is so far from rising in Quicksilver of it self, that till a sufficient Force pulls it up, and permits the Quicksilver to insinuate between the two Plates, the upper is fastned to the lower by that Quicksilver: Tho' upon the first Insinuation of the same it immediately and violently emerges of it self: As Dr. Moor's Famous Trencher did in his Bucket, to his great Surprize; till he was forc'd to solve it by the Introduction of his Spirit of Nature.

Fig.3, and 4. Are Vessels of equal Altitude, but unequal Bases, and of the same Quantity of Water; to shew that Fluids ever press according to their Bases, if their perpendicular Height be equal; and according to their perpendicular Height, if their Bases be equal, whatever Figure they are of.

Fig.5. Is a cubical Vessel full of Water, in order to compute the entire Quantity of the Pressure its Sides and Bottom sustain. And that the Bottom alone sustains the whole Weight of the Water; as is most evident.

Fig.6. Is to shew that each Side of the same Vessel sustains a Pressure equal to half the Weight of the same Water. For since the Pressure at every point, as L, M, N, C, is equal to the Altitude of the Water above it, AL, AM, AN, AC, by erecting equal Perpendiculars LO, MP, NQ, CD, and so at all the intermediate Points, and summing them up, we shall have the Triangle ACD as the Sum of all the Pressures; which being half the Square ACDB, made by as many Perpendiculars equal to the longest CD, and bearing the whole Weight of the Square over it ACDB, shews that the Pressure on every physical Line, as AC of a triangular Prism, and so on the whole Side represented by it, is one half of the whole Water. So that since each of the four Sides sustain half, and the Bottom the whole Weight notwithstanding, the entire Pressure is three times the Weight.

Fig.7. Is a like Method of Computation for an inclined Plain's Pressure, and how to estimate it; viz. by the Weight of Water equal to the Prism represented by the Triangle ARC, where the Lines LO, MP, NQ, CR, are erected perpendicular to AC, and equal to LG, MT, NV, CX, respectively.

Fig.8. Is to determine the Center of Pressure Z against such a Plain; at which if an equal Weight W directly pulls along ZP over the Pulley P, it will just balance the Water, and evenly sustain its Pressure.

Fig.9. Is to shew that this Center of Pressure is no other than the Center of Percussion or Oscillation about an Axis, as D. For the Pressures being as the Perpendiculars EA, FB, GC; and the Percussions, as DA, DB, DC, the Radij of the Circles of Motion; and EA being to FB, as DA to DB; and FB to GC, as DB to DC: The Percussions are still as the Pressures; and so the Center of Percussion, the same with the Center of Pressure.

Fig.10. Is for the Computation of the Quantity and Center of the Pressure on any erect Rectangle under Water; according to that Rule, that the Depth of any Bodies or Surfaces Center of Gravity is to be taken for the perpendicular Altitude of all the Pressures, as a Mean between them.

Fig.11. Is a large Glass Vessel AD, containing Water near the Bottom; with another smaller Vessel FK with Water almost to its Top. There is also a Syphon BHK, with an hollow Stem GH, communicating with both its Legs. To shew that if you stop the Top of the Stem of the Syphon while you pour Oil into both Vessels, a considerable Height above the Bend of the Syphon, and then unstop it, the Oil will press upon the Water in both Vessels, and force it to ascend in each Leg; till meeting at the Bend, it run down the longer Leg, out of the higher Water into the lower. This is to shew how the Air pressing upon Water may raise it up, and cause the known Effects of Syphon, Pumps, Syringes, &c. Which used to be ascribed to Nature's Abhorrence of a Vacuum.

Fig.12. Is a Cube at different Depths of the same Water; to shew how it must have the same Weight in one Place that it has in another, because the Water and Cube have ever the same Proportion of Bulk and Gravity to one another.

Fig.13. Is a Bucket under Water; to shew it can have there no respective Gravity, or cannot preponderate; tho' it has ever the same absolute Gravity.

Fig.14. Are a Bubble and Images of the same Nature, made of Glass, Air, and Water; all so nicely pois'd, that by the Pressure or Relaxation of the Air included, which is done at the Bladder AD, the Bubble and Images rise and fall after a surprizing Manner.