158 Ocean Magnetic Observations, 1905-16 



A ship at sea is never at rest and ever partakes of the motions of the element 

 which supports it. In quiet waters the ship's motions are generally of such a 

 nature that, with the proper instrumental appUances and observational methods, 

 it is possible for a trained observer to make magnetic observations almost as 

 accurately as they are made on land. The instruments are mounted on gimbals 

 (see, e. g., PL 14, Figs. 1 and 5) and all precautions are observed respecting control 

 or elimination of error caused by any lack of precise level of instrument during obser- 

 vations. The observing method then consists chiefly in repetition of observations 

 under varying conditions and for various reversals of instrument, or of magnet, for 

 a period sufficiently long to eliminate the harmonic effects of the ship's motions. 

 This is usually accompUshed in 20 to 45 minutes, the time depending on the instru- 

 ment used and the conditions encountered. 



Ideal conditions for ocean magnetic work are not necessarily periods of calms. 

 During such times a vessel depending chiefly on sail power for headway can not 

 hold a steady course. In consequence, frequent and rapid re-settings of instruments 

 are required or else the changes in the headings of the ship must be continuously 

 recorded and allowed for in the computations. 



Under the usual conditions at sea, observations are more or less difficult, owing 

 to the effects produced by the rolling, pitching, and yawing of the vessel. Were 

 one to wait for ideal conditions, many days would elapse between observations, and 

 long stretches, barren of results, would occur in a voyage. Accordingly, instru- 

 ments and methods must be designed and planned to meet, at least, the usual 

 conditions and to secure the accuracy required for both practical and scientific 

 purposes. Instruments should be designed with a view to diminishing the probable 

 dynamic effects in the observational results produced by the ship's motions. 

 Improvements in this direction are possible to a certain extent by avoiding unsym- 

 metrical distribution of mass in magnets about their centers of motion. The 

 practical application of this principle is Umited, however, by the changing values 

 of the magnetic elements as the vessel sails from place to place. 



In order to make observations in all conditions of sea and weather, the instru- 

 ment and observer must be effectively sheltered from storm, direct sun rays, and 

 spray. The stand with its gimbal rings to receive the instrument must be oriented 

 carefully with outer trunnions athwartship. The instrument is finally mounted and 

 leveled while the vessel is in quiet waters. It is then ready for use under sea 

 conditions, as nearly perfect as is at present possible. 



The specimens of observations on pages 212-225 and discussions, pages 

 434-437, will serve to give some idea as to how well the difficulties caused by 

 ship's motions have been overcome. 



All the effects above briefly discussed, whether caused by natural fluctuations 

 or artificial ones introduced by a moving vessel, while all of sufficient magnitude to 

 be taken into consideration, are, in terrestrial magnetism, generally of a subordinate 

 nature to the values of the primary quantities themselves. In atmospheric elec- 

 tricity, however, the fluctuations resulting from both natural and artificial causes are 

 of the order of magnitude of the primary elements measured. The observational 

 difficulties in this subject will be found discussed in a special report (see pp. 361^22). 



