72 



Ocean Magnetic Observations, 1905-16 



In consequence, an attempt was made to determine the coefficients x, y, and z experi- 

 mentally at Washington, by creating an artificial magnetic field, uniform over a region as 

 great as the needle s^\dngs through, the field being regulated and mamtained at a constant 

 value during the period required to make a set of inclination observations. Such experi- 

 ments were carried out by Observer P. H. Dike, who by his experience on board the Galilee 

 had become famiUar with the difficulties attendant upon the observation of inclination 

 with sea dip-cu-cles. The most feasible method to produce a desu-ed field appeared to be 

 by the use of coils of wire, arranged as in the Hehnholtz type of tangent galvanometer— that 

 is, two equal coaxial coils set at a distance apart equal to their radius. Two such sets of 

 coils, each coil mth a radius of 0.9 meter, with theu- axes at right angles, were used at each 

 of two stations; at each station one set of coils was placed with the axis horizontal and in the 

 magnetic meridian for controlling the horizontal component, and the other with the axis 



Table 1Q.— Needle Corrections for Sea Dip-Circle 169 during Cruise III. 

 [After instniment was modified.] 



Station 



Sitka 



Washington . . 



Washington . . 

 San Francisco' 

 Zikawei 



Honolulu 



Yap 



Jaluit 



Callao* 



Apia 



Papeete 



Christchurch . 



Date 



1907.57 

 1906.94 



1908.50 

 1908.40 

 1907.38 



1907.66 

 1907.29 

 1907.81 

 1908.20 

 1907.18 

 1907 . 1 1 

 1907.99 



In- 

 clina- 

 tion 



+74.7 

 +70.5 



+70.5 

 +62.1 

 +45.6 



+40.0 

 + 6.2 

 + 6 

 - 3 

 -29.3 

 -29.4 

 -67.8 



c.n 



.589 



.599 



.599 

 .539 

 .472 



.381 

 .370 

 .346 

 .300 

 .409 

 .390 

 598 



Regular Dip-Needles 



No. 



of 



Sets 



Observed 



No. 1 No. 2 



-2.2 

 -1.2 



-0.2 



+0.1 



+2.3 

 +6.2 

 +4.4 

 +5.5 

 -2.3 

 -1.0 

 -2 





-3. 



+0. 



+3.2 

 -3.2 



-2.C 



+4 



+5.1 



-0.3 



-1.3 



+2.6 



-7.0 



Computed' 



No. 1 No. 2 



-1.5 



-0.4 



+ 1.7 



+2 

 +4 

 +4 

 +5 

 +0 

 +0 

 -5 



-2.6 

 -1.9 



-0.8 

 + 1.4 



+2 



+4 



+4.4 



+4.3 



-0.3 



-0.4 



-6.3 



(0 -Q 



No. 1 No. 2 



0.0 

 +0.3 



+0 

 -1.6 



-0.6 

 + 1.6 

 -0.5 

 -0.1 

 -3.0 

 -1.8 

 +2.1 



-1.3 

 +2.0 



+4.0 

 -4.6 



-5.1 

 +0.8 

 +0.7 

 -4.6 

 -1.0 

 +3.0 

 -0.7 



Mean Needle No. 7, direct and reversed, 

 deflected by Needle No. 8 



No. 

 of 



Sets 



Observed 



Short 

 Dist. 



+ 5.6 

 + 1.2 



+ 3.5 

 + 5 



- 5.3 



+ 9.7 

 + 13.3 



- 1.8 

 V) 



+ 19.2 

 +22.5 



- 4.6 



Long 

 Dist 



+4.9 

 +2.0 



+0.3 

 -0.8 

 -0.3 



+1.2 

 +5.9 

 +7.0 

 +6.2 

 +7.7 

 +9.9 

 -1.1 



Computed' 



Short 

 Dist. 



+ 1 

 + 1 



+ 1.3 

 + 2.2 

 + 4.4 



+ 6.2 

 + 10.6 

 + 11.3 



C) 



+11 



+ 12.2 

 + 7.4 



Long 

 Dist 



0.0 

 +0.2 



+0.2 

 +0.8 

 +2.2 



+3.2 

 +5.9 

 +6.2 

 +7.5 

 +6.0 

 +6.3 

 +3.1 



(.0 -C) 



Short 

 Dist. 



+ 4. 

 - 



+ 2 

 + 3.7 

 - 9.7 



+ 3.5 

 + 2.7 

 -13.1 



« 

 + 7.6 

 + 10.3 

 -12.0 



Long 

 Dist 



+4.9 

 + 1.8 



+0.1 

 -1.6 

 -2.5 



-2.0 

 0.0 

 +0.8 

 -1.3 

 + 1. 

 +3.6 

 -4.2 



Station Instruments' 



Observatory inductor 

 Referred to Chelten- 

 ham inductor 

 C. I. W. inductor 48 

 0. I. W. circle 178 

 Zikawei circle 33 and 

 C. I. W. circle 178 

 Observatory inductor 

 C. I. W. circle 178 

 C. I. W. circle 178 

 C. I. W. circle 178 

 Obser\'atory inductor 

 0. I. W. circle 178 

 Observatory circle and 

 C. I. W. circle 178 



'The computed values were obtained by means of the formulte adopted from the least-square adjustments; those formula are given on page 68. 

 'Values by the station instrument were all reduced to C. I. W. Standard (see p. 77). 'Station was at Goat Island, near San Francisco. 



'Station was at San Lorenzo Island, near CaUao. 'Deflection observations fafled for the short distance at Callao. 



vertical for controlling the vertical component. The current was suppUed from a portable 

 storage-battery, two separate chcuits being used for the coils controlling the horizontal and 

 vertical components. With these arrangements it was possible to observe simultaneously 

 with a standard earth-inductor at one station and with the dip cu-cle under test at the other 

 station for values of inclmation at regular intervals between -f 88° and -88°. The results^ 

 of the experunent were interesting and afforded valuable data for discussion of the correc- 

 tions of several of the circles used during the cruises of the Galilee. The method, however, 

 was abandoned because of the great expenditure of time required, and because the results 

 showed that practically equally good values could be arrived at by careful consideration and 

 discussion of field comparisons, particularly so when account is taken of the changes caused 

 by deterioration of the pivots. For exceptional instruments it was possible to secure some 

 data for the corrections by studying critically the differences exhibited by needles among 

 themselves for the range of inclination encountered. 



Table 19 gives a condensed summary of the observed and computed data for the 

 adopted corrections for the needles of sea dip-circle 169, which was used throughout Cruise 

 III. This table is typical of the reductions made for each n eedle and circle. Inspection 



'C/. Dike, P. H. Experimental investigation of dip-needle corrections. Terr. Mag., v. 14, 1909 (137-146). 



