Specimen Magnetic Observations and Computations 221 



Inclination Observations, August 23, 1913. 



Observations with sea dip-circle. — Form 27, page 222 gives specimen inclination- 

 observations by the direct or absolute method, using sea dip-circle 189, regular dip-needle 5, 

 obser^^ng in all positions of cu-cle and needle, inclusive of reversed polarity of needle.^ 

 Similar observations were made with needle 9. (For values by indirect method, see p. 218.) 



The scheme of observing was: (1) dip with No. 5, B end down; (2) dip with No. 9, 

 B end down; (3) loaded dip with needle 8; (4) deflections, short distance, first half; 

 (5) deflections, long distance, first half; (6) deflections, long distance, second half; (7) 

 deflections, short distance, second half; (8) loaded dip with No. 8; (9) dip with No. 9, 

 A end down; (10) dip with No. 5, A end down. 



As in the case of the loaded-dip observations, the extreme positions of the swinging dip- 

 needle are recorded to nearest degree. For each extreme position, 5 readings are taken. 



The results are given in the summary, bottom of Form 28a, page 220. The values of 

 I by needles 5 and 9, referred to standard, are: -t-65?22 (No. 5) and -H65?07 (No. 9). 

 The mean, 65? 14, agrees within 0?06, or 4', with the earth-inductor value (-t-65?20). 



Observations with marine earth-inductor. — Specimens of inchnation observations and 

 computations for August 23, 1913, with marine earth-inductor 3, are shown in Form 29 

 (earth-inductor observations) and Form 29o (galvanometer readings), pages 223 and 224. 



The scheme of observation followed was as given on page 201. Galvanometer scale- 

 readings similar to the specimen were made for each group of 4 vertical-circle (V. C.) 

 settings, i. e., for each position (a) conmiutator up with gimbal direct; (6) commutator 

 down with gimbal direct; (c) commutator down with gimbal reversed; and (d) commutator 

 up with gimbal reversed. There were thus 3 additional pages of galvanometer readings 

 similar to the specimen for the complete specimen set of / with the earth inductor. The 

 sequence of the additional galvanometer readings is indicated by the numbers in the columns 

 headed "Settings" on the specimen Form 29. The galvanometer readings recorded under 

 the heading r were made while the coil was spun by turning the crank mounted on the gknbal 

 stand (see PI. 14, Fig. 5) m right-hand direction; those recorded under the heading I were 

 made while the coil was spun by turning the crank in left-hand direction. A second series 

 of observations, set II, gave for the same station and date the following values for inclina- 

 tion: For commutator up with gimbal direct and reversed, -f65?23; for commutator 

 down with gimbal direct and reversed, +65?29; or a mean value of -f 65?26. The observed 

 value obtained with the marine earth-inductor from sets I and II was therefore, -|-65?22, 

 which reduced to standard is +65?20; the value obtained with sea dip-circle 189 at the 

 same station is 4-65?19 (see p. 225). 



The determination of the balance correction, i. e., the correction on the vertical-circle 

 setting, *S„, to obtain the vertical-circle setting for the plane of inchnation, S„ can be best 

 shown by giving the computation of the corrections for vertical-circle settings Si and S3 

 (see also p. 201): 



Correction on S^ = S, - Si = (^r^) (S, - Si) = (-^Z±L^ ( + 2°) = -h 1?32 



vfli — di'' ^ — 4 . 7 — 2 . 4/ 



Correction on S3 = S, - S^ = (-j^) (S^ - S^ = ( ~"\J (+ 2°) = + 0?10 



vos — 04/ V — . 4 — 7 . 8^ 



The value for inclination follows unmediately from S,, bearing in mind that the vertical 

 circle is gi-aduated continuously from 0° to 360° in a clockwise direction, as one looks at 

 the face of the circle. The verniers are fixed in position, while the cu-cle bears a fixed rela- 

 tion to the rotation-axis of the inductor coil such that when the rotation-axis of the coil is 

 horizontal the two vernier-readings are 0° and 180°. 



•The reversal of polarity is made by means of a small electric coil, mounted in forward observing-dome. the magnetizing 

 current being, of course, turned on only when no observations are being made. 



