THE MAGNETIC WORK OF THE CARNEGIE. 1 909-1 916. 



GENERAL REMARKS. 



It was intimated on page 15 that with the steady improvement of the instru- 

 mental appliances and observational methods, the chief concern in accurate ocean 

 magnetic work centered in the correct determination of the outstanding effects 

 attributable to ship's magnetism. Hence arose the desire to have a strictly non- 

 magnetic ship. 



When the results of the magnetic observations made on the Galilee, 1905-1908, 

 were finally deduced and the time and cost involved in the satisfactory determina- 

 tion and elimination of the ship's disturbing magnetic effects were considered, it was 

 obvious that it would have been economy to postpone the inauguration of the ocean 

 magnetic work if we had been certain at the outset that a non-magnetic ship would 

 ultimately be provided. This assurance, however, in view of the uncertainties pre- 

 vailing at the beginning, could not be given. 



But now the desired ship — the Carnegie — has been obtained. Before passing, 

 however, to an account of the work done on her since 1909, it will be of interest to 

 review briefly the difficulties involved in making accurate observations on a moving 

 support. 



The accurate determination of the quantities required to define the direction 

 and intensity of the Earth's magnetic field, at points on land where instruments 

 may be mounted on fixed and stable supports, is a comparatively simple matter. 

 Having properly designed instruments, and using approved observational methods, 

 the trained observer's remaining difficulty on land is the adequate elimination of 

 the natural fluctuations and changes taking place in the magnetic elements while 

 he is measuring them. Fortunately, these magnetic fluctuations, except in rare 

 instances, are of a very subordinate magnitude in comparison with the values of the 

 measured quantities; for example, in the value of the horizontal component H of 

 the intensity of the Earth's magnetic field, the fluctuations during the period of 

 observation rarely amount to 0.2 per cent of the value of H. Only occasionally, 

 when a severe magnetic storm is in progress, may there be momentary fluctuations 

 amounting to 5 per cent of H. The effect of the changes occurring normally in the 

 Earth's magnetic field may be reduced by the method of observation, and any 

 outstanding portion can be determined with the aid of the data recorded at magnetic 

 observatories. 



The observer at sea must seek not only to reduce or eliminate the effects of the 

 natural fluctuations in the Earth's magnetic field, referred to in the previous 

 paragraph, but he must also endeavor to diminish the more troublesome effects 

 caused by the fact that he is obhged to make his observations with a swinging 

 instrument mounted on a moving and unstable support. Fortunately the observer 

 aboard the Carnegie does not also have to contend with the difficulties introduced 

 by a magnetic ship, as did the observer on the Galilee. 



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