_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.
Figure 1. Is a compound Instrument, to shew, why in a Storm the Mercury in the Barometer vibrates so much, by a parallel Case in an Imitation of such a Storm. A A is a large hollow Brass Sphere, into which by the means of the Syringe in _Fig. 2_. Air is crouded till it is 8 or 10 times as dense as usual. H F and L K are Two Barometers, with their Basons in the Boxes F F, K K, which Boxes communicate by a long hollow Tube I I. E E is a Brass hollow Tube, to convey the crowded Air near the Surface of one of the Basons of Quicksilver, which Air passes out of that into a larger hollow Pipe G G, and so into the open Air. Upon the turning of the Stopcock C to give vent to the condensed Air, it rushes with great Force along the hollow Pipes E E, G G; and as it passes not far off the Surface of the Bason of Quicksilver F F, it causes the Mercury in both the Barometers H H, and L L, to descend and vibrate several Inches, as the great Storm made Barometers descend and vibrate in Chambers at a distance from it.