0 30 00 1000 28 91 2000 27 86 3000 26 85 4000 35 87 5000 feet 24 93 1 mile 24 67 2 20 29 3 16 68 4 13 72 5 11 28 10 4 24 15 1 60 20 0 95 25 0 23 30 0 08 40 0 012 Upon these Suppositions it appears, that at the height of 41 Miles the Air is so rarified, as to take up 3000 times the Space it occupies here, and at 53 Miles high it would be expanded above 30000 times; but it's probable that the utmost Power of its Spring cannot exert it self, to so great an Extension, and that no part of the _Atmosphere_ reaches above 45 Miles from the Surface of the Earth. This seems confirm'd from the Observations of the _Crepusculum_, which is observ'd commonly to begin and end when the Sun is about 18 Degrees below the Horizon; for supposing the Air to reflect light from its most rarified Parts, and that as long as the Sun illuminates any of its _Atoms_, they are visible to an Eye not intercepted by the Curvity of the Earth, it will follow from _Fig. 5. Plate 1._ that the proportion of the height of the whole Air, to the Semi-diameter of the Earth, is much about, as 1 to 90, or as the excess of the _Secant_ of about 8½ Degrees to the _Radius_. For if _E_ be the Eye of the Observer, _S_ a Place where the Sun sets at the end of Twilight in _E_, and the Arch _ECS_, or _TCA_, be found 18 Degrees, the excess of the _Secant_ of half thereof _ECH_, would be the height of the Air, _viz._ _GH_: But the Beam of the Sun _ASH_, and the Visual Ray _EH_, do each of them suffer a Refraction of about 32 or 33 Minutes, whereby being bent inwards from _H_ towards _G_, the height of the Air need not be so great as if they went streight; and having from the Angle _ECS_ taken the double Refraction of the _Horizontal Ray_, the half of the Remainder will be 8½ Degrees _circiter_, whose _Secant_ being 10,111, it follows, that as 10000 to 111, so the Semi-diameter of the Earth supposed 4000 Miles, to 44,4 Miles; which will be the height of the whole Air, if the Places _E_, _S_, whose visible Portions of the _Atmosphere_ _ERZH_, and _SHKB_, just touch one the other, be 18 Degrees asunder. At this height the Air is expanded into above 3000 times the space it occupies here, and we have seen the Experience of condensing it into the 60th part of the same Space, so that it should seem, that the Air is a Substance capable of being compressed into the 180000th part of the Space it would naturally take up, when free from pressure. Now what Texture or Composition of Parts shall be capable of this great Expansion and Contraction, seems a very hard Question; and which, I suppose, is scarce sufficiently accounted for, by comparing it to Wool, Cotten, and the like springy Bodies.