Geographic Positions at Sea 227 



ship heading N 22° E, there is no leeway. The true course is the compass course cor- 

 rected for the foregoing, and is entered in the column under "True Course"; it is counted 

 from north as zero, through east, continuously to 360°. The readings of the log and their 

 differences, or the distances in miles, are given in the next two columns. Under the letters 

 N, S, E, and W, is the ordinary form for computing latitude and departure. The depar- 

 tures are converted into minutes of longitude and placed in the last column. 



The lower portion of Form 42 contains the geographic positions and the latitude and 

 longitude increments due to the vessel's run, taken from the final columns of the upper 

 portion of the form. Here, also, the first entry is the last position of the day before. The 

 various increments of latitude and longitude are transferred to their proper places as soon 

 as they become available, and the vessel's approximate position is therefore practically 

 always known by carrying on the summations. As soon as they become available the 

 astronomic positions are written in after the words "By obs." The adjusted values are 

 also written on the same line, but are distiaguished from the astronomic positions by 

 inclosing them in parentheses. As the adjusted positions are not inserted on the form 

 before the dead reckoning has been made, they do not interfere with the legibiUty of original 

 computation. 



The positions of the various magnetic stations at sea are adjusted for the discrepancies 

 that are usually found between the dead-reckoned and the astronomic determinations 

 that immediately follow them. Since the latitude and longitude are rarely obtained 

 sunultaneously except at twilight, they must be adjusted separately and between different 

 control observations. Thus in the specimen Form 42, starting from the star determination 

 at 7^ 04" of the evening of August 22, it will be seen that there is no control over the dead- 

 reckoned latitudes before the noon observations. The noon position by dead reckoning 

 is 39° 21^6 N, and by observation it is 39° 16^8 N. The difference, 4:8, is distributed 

 proportionally to the elapsed time, so that the dead-reckoned latitude of the morning 

 declination station, made at S*" 38", is to be decreased from 38° 51(2 N to 38° 48! 2 N. 

 Similarly, the longitude of the same magnetic station is adjusted by decreasing its dead- 

 reckoned longitude from 39° 49^2 W to 39° 48(4 W by distributing the difference, i:0, 

 between the dead-reckoned longitude and the observed longitude at 8'' 03" in the morning, 

 proportionally to the time elapsed since 7^ 04" of the evening before. 



The position of the observations of magnetic dip and horizontal intensity, made at 

 2h 42" in the afternoon, is adjusted for latitude between the noon position and the two-star 

 position obtained at 7^ 15" in the evening, and for longitude between the morning astro- 

 nomic longitude at 8*" 03" and the afternoon longitude at 4*" 00". The longitudes require 

 a further correction, as indicated in the footnote of the form. This correction, however, 

 can not be determined before the next port is reached, where standard time is available 

 for controlling the chronometer rates, which have to be assumed, in the meantime, as being 

 the same as determined at the last port. This longitude correction in the specimen Form 

 42 depends upon the chronometer corrections determined at St. Helena Jime 24, 1913, 

 and agam at Falmouth September 12, 1913, that is, from a period of 80 days. With the 

 corrections determined at St. Helena and the adopted rates while at sea, the computed 

 Greenwich mean time was found to be 7!3 slow on the standard at Falmouth. Therefore 

 the correction to the longitudes of August 23, 1913, expressed in minutes of arc, is 



-j- X or) = 1(4, increasing west longitudes. 



The final geographic positions of magnetic stations on August 23, 1913, are, therefore 

 38° 48' N and 39° 50' W for morning magnetic-declination results; 39° 29' N and 39° 51' W 

 for magnetic intensity and dip results; and 39° 44' N and 39° 50' W for the afternoon 

 magnetic-declination results, as appear on the magnetic-observation sheets of this date. 



