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.
Hitherto I have only consider'd the Air and Atmosphere, as