. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . allowance fortemperature and pressure, but for such work as may be donewith an ordinary transit the values given in the following tablewill suffice. Angular diameter of sun. The suns angular diameter isabout 0° 32^ With the comparatively high power telescopesnow generally used on transits, this fills a large part of the fieldof view and it is impossible to accurately bisect such a large APPENDIX. 607 MEAN REFRACTIONS [bESSEL] TRUE FOR BAROMETER AT , TEMP. 48° F. Alt. 0° 010
. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . allowance fortemperature and pressure, but for such work as may be donewith an ordinary transit the values given in the following tablewill suffice. Angular diameter of sun. The suns angular diameter isabout 0° 32^ With the comparatively high power telescopesnow generally used on transits, this fills a large part of the fieldof view and it is impossible to accurately bisect such a large APPENDIX. 607 MEAN REFRACTIONS [bESSEL] TRUE FOR BAROMETER AT , TEMP. 48° F. Alt. 0° 01020304050 1° 01020 Refr. 34 54 32 49 30 52 29 03 27 23 25 50 24 25 23 07 21 56 Alt. 304050 030 030 030 Refr. 20 51 19 52 18 58 18 09 16 01 14 15 12 48 11 39 10 40 Alt. 030 030 030 030 0 Refr- 9 469 028 237 497 20 6 536 306 085 49 Alt. Refr. Alt. Refr. Alt. Refr. 9° 30 5 32 18° 2 56 j 30° 1 40 10 0 5 16 19 2 46 35 1 22 11 0 4 48 20 2 37 40 1 09 12 0 4 25 21 2 29 45 0 58 13 0 4 05 22 2 22 50 0 48 14 0 3 47 23 2 15 60 0 33 15 0 3 32 24 2 09 70 0 21 16 0 3 19 26 1 58 80 0 10 17 0 3 07 28 1 48 90 0 0 angular width especially as the apparent motion of the sun acrossthe field of view is very rapid. It therefore becomes advisable(when sighting directly at the sun with the transit telescope) tosight the cross wires on the edges of thesun, as shown in Fig. 2, and make dueallowance for the semi-diameter of thesun. The effect of this is to obtain analtitude which differs from the true alti-tude by the angular value of the semi-diameter. The observed azimuth differsfrom the true azimuth by the semi-diameter ^ cos h. When the sun is atthe horizon, cos /i = 1, and the allowance equals the semi-diameterboth for altitude and azimuth. For higher altitudes the allow-ance for azimuth is much larger than the semi-diameter, sincethe divisor (cos h) is small. If several observations are takenwithin a short interval, the change in this allowance for azimuthduring this short interval m
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