An Explication of the Third PLATE.
Figure 1. Is a large strong Glass Receiver, or Condenser, Arm'd with Brass
Circles at both Ends, and fit to receive and bear the Pressure of Air
considerably condens'd, when crouded into it by a Syringe fitted for that
Purpose. It has also annexed to it a Gage C D, to determine the Quantity
of the condens'd Air within. This Gage consists of a hollow Tube,
Hermetically seal'd at D, with another smaller included, open towards D,
and Hermetically seal'd at the other End. In this smaller Tube is left a
little Quicksilver: This Quicksilver is by the Air at D in the larger
Tube, which communicates with the condens'd Air in the Receiver it self,
and so is of the same Density with it, crouded inwards towards C every
time of the Admission of new Air; and by its whole Length from the End
near D, compar'd with its Distance from the End near C, it determines the
Proportion of the Density of the included Air to that of the common Air.
_Note_, That the Syringe to be made use of with the Receiver, is the same
with that represented in the next Figure, as joined to the condensing
Engine it self; and acts by pulling up the Sucker above the Hole H, for
the Admission of a full Cylinder of common Air, and then crouding it down
into the Receiver; at the Bottom of this Syringe is a Valve, that hinders
what is once crouded in from returning back again, as is necessary on all
such Occasions.
_Fig. 2._ Is the usual Brass Condenser it self, with a Stop-cock E F near
it; to be interposed between the Syringe and the Receiver upon Occasion.
The Instrument, besides the Frame, is composed of a Recipient of Brass,
made of Two Hemispheres, or what is equivalent to them, closed together by
a Ring of wet Leather, to keep in the Air; and because in this Case the
dense Air within endeavours forcibly to disjoin these Hemispheres, they
are confin'd down close by a strong Piece of Iron, and Screws belonging
thereto. The Syringe already describ'd, is represented as join'd to it
after the same manner that it is when the Air is thereby intruded. This
Brass Recipient will bear Air very much denser than the foregoing Glass
one, tho' it being not transparent as the other is, cannot be so pleasant,
nor so well shew the Mutations that happen to Animals or other Bodies in
condens'd Air as the former.
_Fig. 3._ Is the Logarithmick Curve A C _c_, with its Ordinates A B, C D,
_c d_, K δ representing Absolute Numbers, and its Abscissæ, C G or D B,
I _c_ or B d and B δ, representing their Logarithms, whose famous Property
it is, that one Ordinate as A B, is to another Ordinate as C D, or _c d_
or K δ, as that unlimited Space between the Curve and Asymptote above the
one, is to that above the other; and whence is deduc'd the Proportion of
the Air's Rarity at all Altitudes whatsoever; that at 7 Miles high it is 4
times as rare; at another 7, or 14 Miles, it is 16 times as rare, and so
for ever, in a Geometrical Proportion of Rarity, compar'd with the
Arithmetical Proportion of its Altitude; tho' all this is here upon the
Hypothesis that the Distances are not so great, that the real Gravity of
the Parts be sensibly diminished. For in that Case,
_Fig. 4._ Gives the Scheme, which is made use of to discover the Air's
Rareness, even at such Distances, as imply a considerable Alteration in
that Gravity; whence it will appear, that the Density of the Air is
diminished in that Case more than 4 times for every 7 Miles of Altitude.
[[Plate IIII. ― I. Senex sculp.^t]]