154 Philippine Journal of Science 1920 
The theory is given, for example, by Young,‘ who shows oh 
if the observations give an altitude so for the sun a ¥ 
stage of twilight, then the sensible reflecting power of the bes 
mosphere vanishes at a height of about 40 miles, taking acco 
of refraction. But this, he says, is lower than the limit given 
by the ignition of meteors, about 100 miles. 
Wegener * says: 
Most observations of this sort have been made on the principal 
arch, and give, as previously mentioned, a height of 70 km. for oe Hi ina 
layer here considered. The individual values are brought together paren 
following table; the numbers give the angular depression of the sun 
F : ises 
moment at which the twilight arch just sinks below the horizon or 1 
above it. 
TABLE 1° igor 
Schmidt (Athens) 1 5. 6° 
Behrmann (Atlantic) 1 ‘“ 0° 
Bravais (France) 16. 6° 
Hellmann (Spain) 17. 3° 
Liais (Atlantic) 175° 
Moller (Atlantic) 75° 
Bailey (Arequipa, Peru) z a 
Miethe and Lehmann (Assouan) 16. ri 
Carlheim-Gyllenskjéld (Spitsbergen) 17.7 
On a critical consideration of these numbers it becomes very agen 
that most of them are affected by a not unimportant systematic ert ; 
for the vapor laden lowest air strata, lying yet in shadow, cover the big 
edge of the twilight arch when it is nevertheless above the horizon. Int 
connection it is very instructive to see that observations in the Ae 
when the lowest layers are usually more transparent than in the evening: 
give much more accordant results, as the following table shows. 
Evening ser atigaeod 
Spain 15° 20’ 17° 62 
Assouan 14 54 17 «21 
- Atlantic 
18. 182>.. A122 
we 
Consequently, if we assume about 17.4° as the most probable value, 
get by approximate cal. 
jmit 0 
: culation a height of 74 km. for the upper limit 
the light reflecting layers. 
The nature of the systematic error is apparent when one con 
siders that the sunli 
ght received by the eye has first ee 
the atmosphere, passed obliquely downward to tangency at a 
earth’s surface, then obliquely upward to high regions where it (3 
reflected or deviated obliquely downward again through the 4 
‘ Young, C, A., General Astronomy (ed. 1898) 67-69. 
egener, A., Zei 
ty e ” e Le 
tschr. f. anorg. Chem. 75 (1912) 112; Beitrag’ ; 
Geophysik 11 (1912) 104; and, somewhat fuller, Phys. Zeitschr. 12 (191 
170-178 and 214-999. 
*This is Table 1 of the present paper, not so numbered by Wegene?: 
