ALTAZIMUTH SOLAR OBSERVATIONS. 327 
immediately obtained with the zenith distance as argument. In 
computing therefore the results given in the table the augmenta- 
tion has been taken into account ; the geocentric semidiameter is 
taken as 16’, reduced barometric pressure 29-6 inches, i.e. at 32° 
Fahr. and air temperature 48-75 Fahr.; these being the pressure 
and temperature of no correction in Bessel’s refractions. This 
reduced contraction will hereafter be denoted by c’. 
TasLe VII.—Vertical Contraction of the Sun’s Semidiameter. 
S, = 16. Bar. 29-6 in. Therm. 48-75 Fahr. 
Zenith Distances Corrected for Refraction. 
Centre of Sun G- 1b* 30" a be 60" 65° “70° 
bc ae 023 0:24 0-32 0-46 0-62 0-78 1:03 1:44 2:19 
Ditto, Lower 0:23 0-24 0:32 0-46 0°63 0-79 1:05 1:47 2°25 
ogee Toca 0-01 0-0) 0-02 003 0-06 
Zenith Distances Corrected for Refraction. 
Centre of Sun 75° 78° 80° 81° 82° 83° 84° 844° 85° 
pL gl 3:72 5°57 7-67 9-19 11-20 13-90 17 61 20-00 22-92 
Ditto, Lower 3°84 5-80 8 03 9°65 11°84 14:75 18°79 21°44 24°77 
Difference 0-12 0-23 0:36 9:46 0°64 0-85 1:18 1:44 1°85 
Nor also Table IX. § 19, for contractions with the argument 
Miarent ; Sinids distance. 
Difference 
= 
3 
For other values of the semidiameter, and other barometric and 
thermometric readings, the tabular quantities will require correc- 
tion. If $. denote the semidiameter in minutes, given in the 
ephemeris for the time of observation, and B and r respectively, 
the barometer reading reduced to 32° Fahr., and the air tempera- 
ture in Fahr. degrees, then the true contraction ¢ may be found 
from the tabular contraction c’ by the following formula, the last 
two factors of which also approximately express the barometric 
and thermometric correction of the refraction, 
say S, . 461-75 (29)! 
Below 85° zenith distance the sun’s form is generally so irregular 
Pe ea ha one 
* The ratio $ o/16 as a factor is not rigorously accurate but will never-— 
88 not involve an error of 0°01” even at 85° zenith distance: 
