320 Ocean Magnetic Observations, 1905-16 



will suffice for the portion of the cruise to Cape Town to make swings at the most southerly port 

 and at one intermediate between Gardiners Bay and the southerly port selected. In the absence 

 of further instructions, the same considerations apply to future portions. 



2. Declinations will be obtained on each heading, both with CI and D3, especial care being 

 taken with respect to level of latter. A swing with both helms is required separate from that for the 



other elements. 



3. Inclinations— One helm, absolute dips D. C. 189, needle 10 (or No. 9 in case it should prove 

 more satisfactory), readings on each heading being of same extent as for course observations, in- 

 clusive of reversal of polarity of needle; other helm, deflected dips will be obtained in connection 

 with deflections under 4. The same precautions regarding level of instrument apply as in course 

 observations. 



4. Total intensities.— One hekn, making only deflection observations, D. C. 189, needles 7 and 8, 

 using long distance only but having face of needle 7, D and R, each heading. (If swing is repeated, 

 and short distance is available, then use it for deflected dip and total intensity, and on swing with 

 other hebn, use the regular dip needle, e.g., No. 9, not employed on first swing.) 



5. Horizontal inte7isities.— Both hehns, sea deflector 3, using on one swing magnet 45, distance 

 3, and on the other, 2L, distance 1, on each heading. At least two minutes will be allowed for the 

 deflected card to come to rest, and invariably the usual four positions will be taken on each heading, 

 making from 3 to 5 readings of card on each position, according to circumstances. The aim should 

 be not to extend the set beyond 12 minutes, if possible. Precautions must be taken to assure satis- 

 factory condition of instrument and mamtenance of deflection distances. (If swing be repeated, use 

 magnet 45, distance 1, one helm, and magnet 2L, distance 3, other helm.) 



Directions of August 30, 1910, for Shore Magnetic Work.' 



A. Use the earth inductor at all primary shore stations ("primary" in the sense of stations 

 where instrumental constants are determined), rather than at only a part of them. As there are 

 available for observational work but 3 observers, it is desirable to establish for the purpose of constant- 

 determinations not more than 3 stations at the primary ports: A, B, and C. The 3 stations should 

 be chosen so that 2 of them will be on Unes of known azimuth from one of them (in cases where 

 good marks are not available, use might be made of temporary ones, set, however, at least 500 feet 

 distant from closest station and suitably marked to prevent chsplacement or loss during the period 

 of work). Designating the successive days of observation as 1, 2, 3, 4, and so on, dropping out 

 Simdays and hoUdays, the following program will suffice: 



1. Location of station site. Local-disturbance tests by means of dip-circle compass 201, 

 taking magnetic bearings of the same line at various points over site. Selection of station 4, 

 and marks. Azimuth and latitude work at A ; measurements of horizontal angles. Selection 

 of stations B and C. Descriptions of stations. Setting up of tents. 



2. Magnetometer 4 at A, 2 at C (3 complete sets with each magnetometer, a set con- 

 sisting of D, H, H, D, with deflections at 2 distances) ; sea deflector at B. 



3. Magnetometer 2 at A, 4 at C (3 complete sets with each magnetometer); sea deflector 



at 5. . . „. 



4. Magnetometer 2 at ^1, 4 at B (3 complete sets with each magnetometer); marine colli- 



mating-compass at C . . , t •., i. r ji i 



5. Earth inductor at A; dip circle 201 at B (4 determinations of I with each ot needles 1 

 and 2, or with any other pair selected and simultaneous observations with earth inductor). 



6. Earth inductor at B; dip circle 201 at A (4 determinations of I as on day 5). 



7. Earth inductor at B; dip cuxle 189 at A. • , , ji r x. 



8. Same as 7. On days 7 and 8, 4 observations for I are to be made with each needle of the 

 pair regularly used, and 3 to 4 determinations of the total-intensity constants of the regularly 

 used intensity pair; simultaneous observations with earth inductor. 



9. Earth inductor at B; dip circle 203 at A (2 sets for I to be made with each needle-pair 

 for use in emergency with this instrument, and 2 determinations of intensity constants for the 

 pair to be used in emergency; simultaneous observations to be obtained throughout with the 

 earth inductor). ^ 



'See also later directions, pages 321 and 322. 



