202 Ocean Magnetic Observations, 1905-16 



At times the successive detenninations agree exactly ; more frequently they differ by 

 a few hundredths of a degree, and occasionally by three or four tenths of a degree. The 

 causes of the occasional large variations have not been wholly determined. The earth- 

 inductor observations are made on nearly every day the dip circle is used, sometimes before 

 and after, and sometimes in the middle of the dip-circle work, so that the earth-inductor 

 values and those of the dip circle apply to the same tunes and positions. Experiments 

 have been made on board the Carnegie to detennine the relative accuracy of successive 

 determinations for inclination with the earth inductor, by continuing observations through- 

 out a long period of time; in general the indications from such observations have been 

 satisfactory. Thus of six determinations, all applying to the same position and time on one 

 day, the extreme difference ffom the mean in the case of one value was 0?25; of a second 

 value it was 0?18, while the other values differed by less than 0?1 from the mean. Another 

 day's work gave eight determinations, none of which differed from the mean by as much 

 as 0?1. On both days the ship was becalmed, so that the only motion was rolling and the 

 change in heading with the drift was slow. The conditions, therefore, were rather favor- 

 able for earth-inductor observations, since it is possible to maintain the orientation of the 

 inductor in the magnetic meridian by shifting, simultaneously with the change in the ship's 

 heading, the rotating ring of the gmibal stand without intermpting or affecting the observa- 

 tions.^ It is hoped that the mean values of the magnetic inclination determined at sea 

 with the inductor may be depended upon finally to within an absolute accuracy of 3 

 minutes of arc. 



MOUNTING OF MAGNETIC INSTRUMENTS ON THE CARNEGIE. 



At A, the middle point of the bridge (see Fig. 13 and PI. 9, Fig. 2), is mounted in its 

 binnacle the marine collimating-compass, the chief instrument for determining the magnetic 

 declination. 



12 5 



Fig. 13. — Showing Relative Positions of Magnetic Instruments on the Carnegie. 



In the forward observatory, at B, the sea dip-circle was mounted until September 

 1912 on a Dover gimbal-stand, similar to the one at E, which was used for the atmospheric- 

 electric observations from 1909-1914; since September 1912 the sea dip-circle (or the marine 

 earth-inductor) is mounted on the special reversible gimbal-stand (see pp. 196-197). At 

 B are made, accordingly, the observations for determining the magnetic inchnation, I, and 

 the total intensity, F, of the Earth's magnetic field. The horizontal intensity, H, is com- 

 puted by means of the formula H =Fcos I. 



The sea deflector is mounted in its binnacle in the after observatory, at C. With it 

 are determined the horizontal intensity and the magnetic declination. The readings of 

 ship's headuig, which are made simultaneously with the i7-observations at C, were taken 



'The same conditions would be unfavorable for use of the sea dip-circle, since for that instrument it is not practical to shift 

 the rotating ring of the gimbal stand during the observation;'. 



