Methods of Work on the Galilee 15 



this launch, the ship was pulled around, during a swing, or towed along, if need be, in calm 

 weather, in order to get sufficient headway to steer. This was tried with some success 

 on the second and thkd cruises when cabi weather was encountered. 



Regarding the magnetic observations made on the ship's course, the endeavor was to 

 distribute the observations over varying courses as far as possible. In other words, the 

 attempt was to vary the magnitude and sign of the deviation corrections between successive 

 swings as much as possible under the conditions encountered. 



Upon arrival at port, besides harbor swings, shore observations were made, both with 

 the set of absolute land magnetic instruments (magnetometer, and dip circle or earth 

 inductor) and with the ship magnetic instruments, consisting of a standard compass, a sea 

 dip-circle, and a sea deflector described later. Wherever there was a magnetic observa- 

 tory, as at Christchurch (New Zealand), Honolulu, Apia, Zikawei, Sitka, and Tokio, 

 comparisons were made with the observatory standards. Thus sufficient opportunities 

 were afforded for the requii-ed control of the instrumental constants. 



It was soon shown that, by the methods employed, the observational errors were not 

 only considerably less than the chart errors, but were also, in general, less than or about 

 on the order of the errors of the deviation-corrections. In other words, the uncertainty of 

 the deviation-correction soon became our chief concern. If this was so with the precau- 

 tions taken on the Galilee, having, as already said (p. 7), smaller deviation-coefficients 

 than any other vessel previously engaged in magnetic work, and swinging ship every third 

 or fourth day, it would have been much truer on a vessel having larger deviation-coeffi- 

 cients and less opportunity for swings, as had been the case in previous expeditions. 

 Obviously, then, there was no need to defer the effort to obtain useful results until ocean 

 instruments had reached the same state of perfection as land instruments, unless one were 

 assured that funds would soon be available for the building of a wholly non-magnetic 

 ship. This assurance we did not have when the work was begun on the Galilee, and so 

 we were determined to make the ocean work a success and to make the results known 

 promptly. As a consequence, it was abeady possible in the June 1906 issue of Terrestrial 

 Magnetisvi and Atmospheric Electricity to call attention to large systematic errors in the 

 Pacific Ocean charts of the magnetic dechnation, inclination, and intensity of field. Sub- 

 sequent data have been supplied with equal promptness to the leading hydrographic 

 estabUshments which construct and issue magnetic charts for the use of mariners. 



The prompt reduction of the observations and the many controls insisted on whenever 

 the vessel reached port served to disclose the weak points, but not always as quickly in the 

 early work as desired. Thus, because the deviation-coefficients were different at the various 

 positions of the instruments, it was not possible to get an immediate comparison, for 

 example, in declination observations made at two different stations on the ship. The devia- 

 tion corrections could not be successfully determined until the completion of a cruise cover- 

 ing a large enough range in magnetic latitude. And here is where the great advantage of 

 having a non-magnetic ship, hke the Carnegie, counts most heavily; on board her it is 

 possible to make a nearly final computation a few minutes after completing the obser- 

 vations, and thus to check up an observation at once and repeat it, if necessary. 



The observers did not merely make the observations on board the ship, but also the 

 first, or field, reductions. The observations and preliminary computations were not allowed 

 to accumulate, but were forwarded promptly from the first maihng-port to the office at 

 Washington, where they were subjected to a careful examination, and the final reduction 

 was made as soon as possible. Abstracts of the chief results obtained were kept aboard 

 for future guidance. 



At each cable port the commander of the vessel reported his arrival and experience, 

 and held the departure of the vessel subject to advice from the Director. Thus the ship 

 was kept in close and effectual touch with the Director throughout, and possible improve- 



