320 G. H. KNIBBS. 
at any zenith distance when moderate accuracy only is required, or 
when the differences of the corrections only have to be determined. 
Tape IIJ.—Correction of the Refraction of the Observed Mean 
Zenith Distance for a semidifference of 1°. Barom. 29-6 Therm. 
48°75 Fahr. 
Appt. Zenith Distance 45° 50° 55° 60° 65° 70° 75° 
Oorr. to Refract. +. -03” -05’ -08” -12” +21"° 39" “OF 
Appt. Zenith Distance 773° 80° 81° 82° 83° 84° 85°. 
Corr. to Refract. + | 1°50” 2-71” 3-63” 4:86” 6:78" 9-77” 14°65" 
The correction for any other semidifference of apparent zenith 
distance may be approximately found by multiplying the tabular 
value by the square of the semidifference expressed in degrees and 
decimals of a degree. For any other pressure B, and air temper- 
ature 7, the tabular quantities should be multiplied by factors 
expressing the barometric and thermometric corrections to the 
tabular refractions. These factors! are 
Bo EIS (n) 
296 41347 ree 
The corrections for the error, which it is necessary to apply to 
the azimuthal angle measured from the elevated pole, and to the 
hour angle measured from the meridian toward east or west will 
be respectively 
q= —~6— (16) 
& = + 6, sin (906 9,......6605: (17) 
as it is easy to see from (g)in § 4: 6 of course denotes declination. 
7. Diurnal Aberrution.—The orbital motion of the earth causes 
a displacement, called the annual aberration, of the position of 
the sun and stars; which being independent of locality on the 
terrestrial surface, is taken cognisance of in the apparent right 
ascensions and declinations tabulated in ephemerides. The diurnal 
ir tamper- 
temper 
The coefficient 461-75/413 +7 approximately expresses th 
ature factor of Bessel’s refractions. In its stead the factor T in Chamber’s 
Mathematical Tables, p. 431, Edit 1885, may be used with advantage. 
For 0° Fahr. it is 1106, for 100° Fahr. 0°909. 
