16,2 Fisher: Low-Sun Phenomena in Luzon 161 
the sunlight, which passes overhead at the moment of passage 
of the blue dawn, has been refracted and its lowest rays have 
been tangent to the sea surface some 700 sea miles east of Manila, 
and have suffered refraction nearly equal to twice the horizontal 
refraction at the point of tangency. On account of the near 
constancy of pressure and temperature conditions east and west 
in the Tropics, the most scientific way would be to observe the 
sun rising from the ocean and thence compute the refraction. 
As this is impossible, I have had recourse to monthly and yearly 
means of pressure and temperature at Manila, and have com- 
puted mean values of the horizontal refraction from them, hoping 
that the effect of land may be partly neutralized by the nearness 
of Manila Bay and Laguna de Bay. 
Table 4. 
Month. 
Pres- 
sure.a 
Shade 
temper- 
ature.a 
Relative 
humid- 
ity.a 
Vapor 
pres- 
eure.a 
Horizontal 
refraction. 
Pulko- 
wa. b 
Radau.c 
mm. Hg. 
»C. 
Per cent. 
mm. 
January 
761. 15 
24.9 
78.1 
18.1 
32.1 
31.4 
February 
61.25 
25.3 
73.9 
17.6 
31.9 
31.4 
March 
60. 54 
26.6 
71.6 
18.1 
31.3 
31.1 
April 
59. 42 
28.1 
69.7 
19.4 
30.7 
30.8 
May 
58.35 
28.4 
76.0 
21.6 
30.4 
30.7 
June - 
67.92 
27.9 
80.8 
22.3 
30.4 
30.9 
July. 
57.24 
27.0 
84.8 
22.4 
30.4 
30.9 
August 
57. 33 
27.0 
85.0 
22.4 
30.5 
30.9 
September 
57. 42 
26.8 
85.8 
22.4 
30.7 
30.9 
October 
56.86 
26.7 
83.7 
21.6 
31.4 
31.0 
November 
59.36 
25.9 
82.6 
20.3 
31.8 
31.2 
December 
60.35 
26.2 
81.3 
19.2 
32.1 
31.4 
Year 
759. 08 
26.6 
79.4 
20.4 
31.37 
31.04 
“ Averages over the period from 1885-1916, taken from the Annual Report of the Weather 
Bureau (1916). 
Computed from the abridged Pulkowa refraction tables contained in Campbell’s Elements 
of Practical Astronomy, second edition. 
' Computed from the refraction tables contained in Connaissance des Temps (1917). It will 
be noted that the Radau values are "more uniform than the Pulkowa. In computing I use 
the former. 
The pressures given in Table 4 are reduced to standard grav- 
ity before computing by subtracting from each 1.72 millimeters. 
A formula for the calculation is derived as follows : In fig. 2, 
0 is the earth’s center, R its radius, Z the zenith of the observer, 
ST the direction of a ray before refraction, TV its direction after 
refraction, r the horizontal refraction; so that the angle ZTV, 
the deviation of the ray, is 2r. The ray of course passes in a 
