318 Ocean Magnetic Observations, 1905-16 



4. Total intensities. — On second swing, called for in 3, make deflection observations with D. C. 

 189, needles 7 and 8, using both distances, and with face of needle 7, D and R on each heading. 



5. Horizontal intensities with sea deflector 3. — First helm : use magnet 45, and on the other, 2L, 

 at distances 1 and 3 on each heading, obtaining as many sets as possible. (Special care should be 

 taken to see that the instrument is in best condition, and that deflecting magnets are properly placed 

 in position. Sufficient time must be allowed on each heading for the compass card to settle down, 

 and precautions taken to set up as little motion as possible in the liquid.) 



6. The Commander is at liberty to repeat any of the prescribed observations found necessary. 



7. For the present, other places of swings will not be designated. Should a port be found, 

 however, in the Southern Hemisphere, where conditions are suitable, it would be desirable to swing 

 vessel once more towards the end of the present year, or possibly at Cape Town. * ♦ * [The 

 vessel was actually swung in Gardiners Bay (Aug. 31-Sept. 2, 1909; Dec. 15, 16, 1913), at Rio de 

 Janeiro (Dec. 23, 24, 1910), at Falmouth (Oct. 4, 1913), and 8 times at sea.] 



B. Magnetic Observations on Course. 



1. The attempt will be made to secure some magnetic data daily — the three elements, whenever 

 conditions permit. The method of observing each element in duplicate, simultaneously with 

 different instruments and observers, is again to be followed as rigidly as possible. Observers are 

 again to alternate in observing any particular element. It is desirable, whenever conditions permit, 

 to obtain the three elements, as nearly as possible, for the same geographic position, but it is realized 

 that this is not so readily accomplished as far as the declinations are concerned, since for this element 

 the time of observing can not be arbitrarily chosen. The dips and intensities should be observed, as 

 far as practicable, between 2 and 5 p.m., local mean time — in general, between 3 and 4 o'clock; 

 the diurnal-variation corrections at these times will usually be negligible. 



2. Declinations with Cl and D3. — Observations with the latter instrument, if made with care, 

 will afford a check upon the former. 



3. Inclinations. — The observations will consist of absolute dips with needles 9 and 10, D. C. 

 189 and deflected dips, needles 7 and 8, using two distances whenever possible. In the computation, 

 double weight will be given, in general, to each direct dip, and single weight to each deflected dip or 

 double weight to the mean of two deflected dips. Level of instrument on the gimbal stand has been 

 found to be a very important matter; the whole error of level may enter into the dip, and it is not 

 eliminated by the method of observation, nor by taking the mean of several needles. Every oppor- 

 tunity will be taken to control this source of error, and the necessary precautions will be observed 

 regarding heights of foot-screws, etc., as prescribed on the previous cruise. [Beginning at Tahiti in 

 1912, a reversible gimbal-stand was used; see pages 19&-197, and PI. 14, Fig. 5.] 



4. Total intensities. — Always make deflection observations with D. C. 189, needles 7 and 8, 

 using two distances whenever possible. When short distance becomes unavailable, then observe 

 loaded dips, needle 8 (weight 11) two sets, next deflections long distance, face of needle 7, D and R, 

 and close with two sets loaded dips. 



5. Horizontal intensities. — These observations will be made with the sea deflector supplied, 

 using both magnets and both distances, observing precautions noted under A 5; as many sets will be 

 obtained as possible during the dip and intensity observations in forward observatory. Distances 

 1 and 3 will be used until otherwise instructed. Recorder will take simultaneous readings of ship's 

 head with Kelvin compass. 



6. General remarks. — Observers should use every reasonable endeavor to guard against instru- 

 mental changes and should try to ascertain causes of errors immediately upon discovery. They 

 must be careful regarding presence of articles on their persons, or on others close by, Ukely to affect 

 the observations. Whenever the instructions do not exactly fit the conditions to be met, the Com- 

 mander, in accordance with his previous experience, will make the necessary modifications and 

 amplifications. The Commander will also see to it that every care is taken as to holding of course 

 during observations. In order to secure effective control of the work, he, at least once a week, will 

 make a complete set of observations with each instrument; these need not be additional observations, 

 but a part of the regular scheme of observation; in connection with these observations a general 

 report will be made of the condition of instrument. To guard against systematic errors as far as 

 possible, he will exercise a similar control on all computations. 



