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 CD, 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 EF 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 ACc, with its Ordinates AB, CD, cd, Kd representing Absolute Numbers, and its AbscissÆ, CG or DB, Ic or Bd and Bd, representing their Logarithms, whose famous Property it is, that one Ordinate as AB, is to another Ordinate as CD, or cd or Kd, 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.