SPECIMENS OF OBSERVATIONS AND COMPUTATIONS. 



The following specimens of observations and computations, applying to the 

 date August 23, 1913, will assist in making clear the methods followed on the 

 Carnegie, and will serve to illustrate a typical day's observations at sea. The 

 observing conditions wiU be found stated on the forms. The roll of vessel was 

 about 4° starboard to 4° port; hence, the total roll, from side to side, was about 8°. 



MAGNETIC OBSERVATIONS AND COMPUTATIONS. 



Reference to the instruction.s for the magnetic work on Cruise II (pp. 317-322), and to 

 the detailed description of methods followed in the Galilee work (pp. 33-57), will doubtless 

 furnish the information required on any matter which may not be wholly understood from 

 the forms themselves. Specimens illustrating shore work will be found in Volume I, pages 

 30-41. For specimen determinations of instrumental constants, see pages 234-250. 



Declination Observations, August 23, 1913. 



Observations withmarine collimating-compass. — Form 21a, page 213, illustrates the record 

 of observations for magnetic declination made with the marine coUunating-compass (C 1 ) . 

 The specimen gives the first 4 of 20 sets made during a period of 1 1 minutes by two observ- 

 ers, P and S. The scheme of observation calls for 5 sets by one observer, next 10 sets by 

 the other observer, and finally, 5 sets again by the first observer. The mean results of 

 the two observers are therefore comparable, referring as they do to the same tune and to 

 the same geographic position of ship. There are 10 readings in each set; hence, a complete 

 determination by this one instrument consists of 200 readings. The times are noted by 

 watch M, which requires a correction of +9''19'°53, as indicated in the portion headed 

 "Chronometer Comparisons." It should be noted that the standard chronometer rate 

 has been adjusted for sea rate, as was subsequently determined on arrival at the next port. 

 Since this correction affects the longitude by precisely the same amount, the local apparent 

 time of the original computation remains unaltered. 



The scale readings have been taken in this specimen at exact intervals of 3 seconds, 

 called out to the observer by the recorder. The observed angle between the Sun and 

 scale, Ao, is corrected, if necessary, for index error to obtain A of the formula (6) , page 

 181. The altitude of the Sun's estimated center, ho, is measured by a third observer 

 between the fifth and sixth readings of each set. An index correction, 0', and the dip of 

 the horizon, —4' ( — 0?07), are appUed to /)o to obtain h of the formula. 



The observed quantities or their means corrected as above stated are transferred to 

 Form 26 or 26a for computation. Form 26 (p. 215) is used for low altitudes of the Sun, 

 for then the approximate formula (6), page 181, and the corresponding tables are especially 

 suitable. "VVTien, however, the Sim is so high that the interpolation becomes laborious 

 or the limits of the tables are exceeded, then the alternative form (26a) is used. Examina- 

 tion of the latter form, of wliich a specimen is given on page 214 merely to illustrate the 

 use of the rigorous formula for computing the angle A, shows that it is divided into two 

 parts. The upper part contains the means of the observed quantities corrected and 

 arranged in 4 groups of 5 sets, one group for each scale observed by each observer. The 

 means of each group are then transferred to the places indicated in the lower part of the 

 computation. The scale readings are reduced to center by subtracting 5 divisions from 

 each and converting the results into degrees of arc. These quantities are then added 

 algebraically to the scale constants, -f 0?37 and 180?31, and carried down and entered at 

 " constant + a" near the bottom of the form. From the values of m, h, and A the angle A 

 is computed and appUed directly to the values of "constant + a" by which operation the 

 magnetic bearing of the Sun from south around by west is obtained. The astronomic 

 azimuth is computed from any convenient azimuth tables with the arguments, Sun's 

 declination, latitude, and local apparent tune. The difference between this azimuth and 

 the magnetic azimuth is the magnetic declination. 



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