60 Ocean Magnetic Observations, 1905-16 



latitude must be altered by a quantity which depends upon the run of the ship between 

 observed latitude and the place of the magnetic observations. 



The error in run may be controlled by the astronomic observations immediately pre- 

 ceding and following the magnetic observations. This procedure is, in fact, the method 

 employed in the ocean work. But in attempting to assign hmits of accuracy we are again 

 confronted with the error in this control which depends on stability of speed and direction 

 of ocean currents, and upon constancy of leeway and steering. Again, if the observed 

 Sun or star be east or west of the meridian, there is an additional uncertainty introduced 

 by the unknown error in the assumed clironometer-rate. This error, however, need not 

 be considered in the case of the Galilee and Carnegie, since it was controlled by time compar- 

 isons at every port available for the purpose, and was distributed back when appreciable. 

 An investigation of some of the three-star determinations of the ship's position made on the 

 Galilee indicates that if the Sun or star be favorably situated and the weather and sea con- 

 ditions fair, the average error to be expected in the determination of the geographic position 

 is less than 2 miles. The error in the control of the "error of run" is usually insignificant if 

 the controlling astronomic observations are not more than 6 hours apart. This has usually 

 been the case in the Galilee and Carnegie observations, except in high latitudes, where fog 

 and clouds prevail. Of course, there are exceptional times when no astronomic observa- 

 tions are possible for several days. The geographic positions for the results of magnetic 

 inclination and intensity are then more or less uncertain. In the case of magnetic-decUna- 

 tion results, however, the Sun or star that serves for the decUnation observations usually 

 permits of at least a fairly good determination of position. (See PI. 2, Fig. 4.) 



REDUCTION FORMULAE AND DETERMINATION OF CONSTANTS. 



CONSTANTS AND CORRECTIONS FOR SEA INSTRUMENTS. 



The instrumental constants and reductions to standards (see p. 77) of the sea instru- 

 ments used in the Galilee work were determined at Washington and at the various ports 

 visited by comparison with standardized land instruments. The method adopted in these 

 comparisons was that of simultaneous observations, except during the earlier work, when 

 the method of alternate observations was used. In order to refer values of the magnetic 

 elements at one observing station to any of the other stations, station-differences were care- 

 fully determined at each port from the observations with the land instruments, following 

 the methods described in Volume I (pp. 19, 20). 



Decunation Observations. 



Standard compass and azimuth circle. — For specimen declination-observations with the 

 standard Ritchie compass and azimuth circle on board ship, and the corresponding compu- 

 tations, see pages 47-48 and 55. The purely instrumental corrections for the compass and 

 azimuth circle arise from (1) card-graduation error and eccentricity of card mounting, (2) 

 index error, and (3) lack of correct adjustment of the azimuth-circle attachments. Card- 

 graduation errors and index errors were determined at shore stations by observing the mag- 

 netic azimuths of a series of 6 or more marks in the horizon, i. e., at altitude practically 0°, 

 the marks being selected to give as nearly equal angular distribution as possible. The mag- 

 netic azimuths were controlled by simultaneous declination-observations with a standardized 

 magnetometer at a second station. For each instrvmtient the total periodic errors of the com- 

 pass-card readings, determined in this way at a number of stations, were plotted with the 

 total errors as ordinates and card readings as abscissae, and a mean curve was drawn. The 

 mean ordinate of the resulting graph represents the index correction, x, of the compass and 

 azimuth circle, for altitude, h = 0°; the ordinates of the graph referred to a new axis of 

 abscissae at a distance x from the old one are the purely periodic corrections, e. The correc- 

 tions, Aj^orAac, arising from any lack of correct adj ustment of the azimuth-circle attachments 



