Geographic Positions at Sea 



229 



The computation has been made with an approximate latitude 39° 02! 6 N, which was 

 taken from the dead reckoning. This latitude was adjusted later, as has been explained, 

 and consequently the resulting longitude required a correction for the change of latitude, 

 3!7 south. This change is computed from the well-known differential formula or taken 

 from any one of the numerous nautical tables designed for the purpose. The specimen 

 given is one of two sets. The other, havmg been made and computed independently by 

 another observer, gives a corrected longitude of 39° 48^4 W. The mean, 39° 48! 7 W, is 

 the adopted value finally entered on the dead-reckoned sheet after the words "By obs." 



Latitude Observations. 



The latitude observations are recorded and computed on Form 40, of which a specimen 

 is given below. According to the usual practice at sea, the maximum altitude of the Sun 

 is noted at noon. The specimen shows the observations made at noon on August 23 by 

 three observers, C. C. C, H. R. S., and M. C. The mean of three, 39° 16:8 N, is adopted 

 and entered in the dead reckoning. 



■> The longitude is again determined in the afternoon by two observers in the manner 

 already shovm. (Form 41, p. 228). The results by two observers are 39° 49!5 W and 39° 

 50! 7 W, the mean of which, 39° 50! 1 W, has been entered in the appropriate place of the 

 dead-reckoning sheet. 



Geographic Positions at Sea: Latitude Observations 



(Form 40) 

 Date: Sat., Aug. 23, 1913 Vessel: Camegie Com'd'r: W. J. P. Obs'rs and Comp'rs: C. C. C, H. R. S., M. C. 



Observer 



Sextant 

 No. 



Object 



Obs'd Alt. 



Index Corr'n 



Alt. Corr'd 

 for I. C. 



Corrections 



Decimation calculation 



c. c. c. 



H. R. S. 

 M. C. 



22876 

 2617 

 2611 



Q 

 S 

 2 



62 05 00 

 62 04 20 

 62 01 30 



Sextant 

 No. 



Observations for 



Index Corr'n 

 (Sun's diameter) 



22876 

 2617 

 2611 



+30 40 

 31 40 

 34 10 



-1 00 



00 



+2 30 



Mean 



Total corr'n 

 True alt. 

 Zen. dist. 

 Decl'n 



Latitude (N) 



62 04.0 

 62 04.3 

 62 04.0 



62 04.1 



+ 11.3 



62 15.4 



27 44.6 



+ 11 32.2 



Semi-Diam. 

 R. &P. 

 Dip 



Total 



+ 15.9 



- 0.4 



- 4.2 



+ 11.3 



Decl'n at G. M. N. 

 Hourly Diff.: -0'.85 

 Time from G. M. N. : +2^ 7 

 Corr'n: (+2.7)(-0.85) 



Decl'n at Obs'n 



+ 11 34.5 

 -02.3 



+ 11 32.2 



39 16.8 



Position: bridge 

 Height of eye: 18 feet 

 Horizon: good 

 Wind: S 2 

 Barometer: 772 nun. 



Remarks 



Thermometer: 26 ?5 C. 

 Starb. log: 13.8 



Port log: 



Dir. of object: S 



Latitude and Longitude Observations (Two-Star Altitudes). 



The astronomic work of the day is finally completed by two-star observations, both of 

 which are recorded and computed on Form 41a, which is practically the same as Form 41, 

 already described. When one of the observations, however, is a meridian altitude, then 

 latitude form (No. 40 above) is used. In the specimen (p. 230), the observations on Arc- 

 turus and Jupiter by one of the two observers, together with the computations, are shown. 

 The dead-reckoned latitude, 39° 47 !l N, was used in the computations; the correction to 

 this latitude is -|-0!7, making a final value of 39° 47! 8 N, as obtained from differential 

 formulae or from nautical tables. The approximate correction A tp, by differential formulae is 

 obtained from the equation 



X' - X' 



A(p = 



cot A" — cot A 



; cos tp 



in which X' and X" are the approximate longitudes (reckoned westward), determined 

 respectively from the separate stars, by using an assumed latitude, v?, while A' and A" are 

 the respective azimuths reckoned positive from south around by west. 



