C I07 ] 
Upon thefe Suppofitions it appears, that at thehightof 
41 miles ^ xhzAir is fo rarified, as to take up 3000 times the 
fpace it occupies here, and at 5 j miles high, it would be ex- 
panded above 30000 times ; but tis probable that theut- 
moft power of its fpring cannot exert it felf, to fo great an 
exteniion, and that no part of the Atmoffhere reaches above 
45 ;^/7^j' from the furface of the Earth. 
Thisfeems confirmed from the Obiervations of the Cre- 
fufmhm^ which is obferved commonly to begin and end 
when the Sunn is about 18 degrees below the Horizon ; for 
fuppofing the Air to refled light from its moft rarified parts* 
and that as long as the Sun illuminates any of its Atoms^ they 
are vifible to an Eye not intercepted by the Curvity of the 
Earth, it will follow from Fig. 6. that the proportion of the 
hight of the whole Air, to the Semidiameter of the Earthy is 
much about, as i to 90, or as the excels of the Secant of a- 
bout 8 ; degrees to Radius : For if E be the Eye of the Ob-^ 
ferver, S a place where the Sun lets at the end of twilight 
in E, and the Arch ECS, or TCA be found 18 degrees, the 
excefs of the Secant of half thereof ECH, would be the hight 
of the Air viz. GH : But the beam of the Sun ASH, and the 
vifiial Ray EH do each of them fuffer a Refradion of about 
3 2 or 3 3 minutes, whereby being bent inwards from H to- 
wards G, the hight of the Air need not be fo great as if they 
went ftreight ; and having from the Angle ECS taken the 
double Refra^io^ of the Horizontal Ray^ the half of the re- 
mainder will be 8 ' degrees ^-/mV^r, whole Secant 10 
II I it follows that as 10000 to 1 1 1, fo the Semidiameter of 
of the Earth fuppofed 4000 miles, to 44,4 miles; which 
will be the hight of the whole Air, if the places E, S, whofe 
Vifible portions of the Atmofphere ERZH, and SHKB juft 
touch one the other, be 18 degrees affunder. 
At this height the Air is expanded into above 3000 times 
the fpace it occupies here, and we have feen the experience 
of Condenfing it into the 6oth part of the lame ipace, fo 
that it fliould feem, that the Air is a fiibftance capable 
P of 
