An Explication of the Second PLATE.
Figure 1. Is the Air-Pump, with its Receiver and Gage, as ready for Use;
_a a_, _a a_ are two strong hollow Cylindrical Barrels, in which are
suppos'd to be Suckers, with their Handles _c c_, _c c_ notched; into
which Notches a Cog-wheel falls, which Cog-wheel moves upon the Axis _f_,
when the Engine is put into Motion by the Winch _b b_. _g g_, _g g_ are
two Cylinders of Wood, fixed to the Frame of the Air-Pump, with Screws at
the Top, on which the Nuts _e_, _e e_ do run, and press down the upper
Piece _f f_ upon the Tops of the Brass Barrels, to fix them both at Top
and Bottom. _h h_ is a Swan-neck'd, or small bended hollow Brass Pipe,
leading from the Top-Plate _i i i i_, or rather from the Brass hollow
Piece above _n n_, which communicates through that Top-Plate with the
Cavity of the Receiver. This Pipe is screwed to a bottom Brass Piece,
included in the Box _d d_; which is perforated not only lengthways, but
also upwards, in three Places: The Middle one for a Communication with
this Swan-neck'd Pipe, and at the two Ends through small Cylinders;
inserted into the two Brass Barrels _a a a a_; and 'tis by this Threefold
Communication, that the Air is pump'd out of the Receiver. _l l l_ is the
Gage; which is no other than a common Barometer, or Weather-Glass; with
its Bason of Mercury _m m_, fix'd to the Engine by a particular
Contrivance, and its Index or Boxen Receptacle, with Inches, and its Cork
to support that Index upon the Surface of the Mercury, and to rise and
fall with it; for the Exactness of measuring the Height of the Mercury
from that Surface. Only this Barometer is open at the Top, and
communicates, as does the Swan-neck'd Pipe, with the Cavity of the
Receiver. _n n_ is a Stop-cock, that communicates also with the Cavity of
the Receiver, and either excludes or readmits the Air, as you see
convenient. _k_ is the Bottom of the Receiver, ground true to fit the
Brass Circle below it; to which it is affixed by the Hand at first, and
afterward by the Pressure of the Air, with wet Leather instead of Cement.
_Fig. 2._ Is a Barometer Tube, open at the Top H, and included in such a
Receiver G B, as gives room for it to stand upright, and yet permits the
Air to go backward or forward on its Surface, according as you pump the
same out of or readmit the same into that Receiver. And this is done so,
that the included Air C D, which supports the Mercury, by pressing on the
Surface of that in its Bason E D, is confin'd within. This small Quantity
of Air, on the Extraction of that in the Receiver, will, by its
Elasticity, raise the Mercury almost as high as the usual Standard: And
thereby shews, that the Spring of any small Part of common Air presses
equally with the whole correspondent Column of the Atmosphere.
_Fig. 3._ Is a Contrivance to make an Explosion of Gunpowder in Vacuo:
Where H D is a red hot Iron, standing on its Pedestal E, within a Receiver
G C; and F is a Cock made above like a Dish, to contain the Gunpowder;
which by the pulling up and thrusting down a strong Wire, with a Hole like
the Eye of a Needle, is in a certain Quantity let fall every time upon the
hot Iron; and on the Explosion produces Flame, and fictitious Air; but
very little Sound, by reason of the Absence of the Air that should convey
it.
_Fig. 4._ Is a Syringe, which will suck up the Water in the Glass C D,
when it is in the open Air; but will not do the same under the Exhausted
Receiver E F, unless for so small an Altitude as the remaining Air can
sustain.
[[Plate III. ― I. Senex sculp.^t]]