MifceUanea Curiofa. 8 5 



UPON thefe Suppositions it appear?, that 

 at the height of 41 Miles the Air is fo ra- 

 rified, as to take up 3000 times the Space it oc- 

 cupies here, and at 53 Miles high it would be 

 expanded above 30000 times ; but it's probable 

 that the utmoft Power of its Spring cannot exert 

 it felf, to fo great an Exrenfion, and that no 

 part of the Atmofphcre reaches above 45 Miles 

 from the Surface of the Earth. 



This feems confirm'd 'from the Observations 

 of the Crepufculum, which is obferv'd common- 

 ly to "begin and end -when the Sun is about 

 18 Degrees below the Horizon ; for fuppoing 

 the Air to reflect light from its moft rarihed 

 Parts, and that as long as the Sun illuminates 

 any of its Atoms, they are viGble to an Eye not 

 intercepted by the Curvity r.f the Earth, it will 

 follow from Fig. 5-. PUte r. that the proportion of 

 the height of the whole Air, to the Semi-diame- 

 ter of the Earth, is much about, us 1 to 90, or 

 as the excefs of the Secant of about 8£ Degrees 

 to the HijUis. For if E be the Eye of the Ob- 

 server, s a Place where the Sun fees at the end of 

 Twilight in E, and the Arch E C S, or TC A, 

 be round 18 Degree?, the excefs of the S~ cant or 

 half thereof E C 11, would be the height of the 

 Air, v j K . GH-. But the Beam of the Sun' 

 AS » and the Vifcai Ray EH, do each of 

 them fuffer a Refraaion of about 3 k or Mi- 

 nutes, whereby being bent inwards from H 

 towards G, the height of the Air need not be 

 fo great as .f they went rtreight ; and having 

 from the Angle E C S taken the double Refra- 

 fron or the bm k ,„ m i KVt the half of the 

 Remainder will be 8* Degrees circitcr, whofe 

 S»um being IO , i i i, it follows, that as iocoo 

 G 3 to 



