An Explication of the Second PLATE.
Figure 1. Is a large Glass Vessel A D full of Water as high as E F. Within
this is a lesser Glass Vessel P H, open at both Ends, but somewhat
narrower at the Bottom. Through the middle of this goes a strong Wire M N,
to which is fixed at the lower End a Plate of Lead G H, with wet Leather
to its upper Surface, to be applied to the large lower Orifice of the
lesser Glass I K, 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 E F and R S
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 A D, with a small Cylinder of
Wood below G H fixed to its Bottom, and made very smooth at Top; and
another like Cylinder of Wood above G H, 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 E F. 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, A L, A M, A N, A C, by erecting equal Perpendiculars L O, M P, N Q,
C D, and so at all the intermediate Points, and summing them up, we shall
have the Triangle A C D as the Sum of all the Pressures; which being half
the Square A C D B, made by as many Perpendiculars equal to the longest
C D, and bearing the whole Weight of the Square over it A C D B, shews
that the Pressure on every physical Line, as A C 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 A R C, where the Lines L O, M P,
N Q, C R, are erected perpendicular to A C, and equal to L G, M T, N V,
C X, 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 Z P 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 E A, F B, G C; and the Percussions, as D A,
D B, D C, the Radij of the Circles of Motion; and E A being to F B, as D A
to D B; and F B to G C, as D B to D C: 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 A D, containing Water near the Bottom;
with another smaller Vessel F K with Water almost to its Top. There is
also a Syphon B H K, with an hollow Stem G H, 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 A D, the Bubble and Images rise
and fall after a surprizing Manner.
[[Plate III. ― Sutton Nicholls sculp.]]