Extracts from Instructions, 1909-16 321 



B. The above program, at primary stations, calls for the services of 3 observers on days 1, 2, 3, 

 and 4. On days 5 and 6, the third observer might complete azimuth or latitude work, and com- 

 pute the work of the first fom- days. At the ports where swing observations are to be made, days 

 7, 8, and 9 might be devoted by the third observer, using magnetometer 4 and dip circle 201, to 

 observations at stations suitably selected for determining the distribution of magnetism in the region 

 of the proposed swing. The order of program may be varied to suit the conditions encountered. 

 The work with 203 (the reserve sea dip-circle) may be omitted at every other station. 



C. To determine the distribution coefficients for the land magnetometers, it will suffice to make 

 deflections at 3 different pairs of distances at the 3 stations. In no case, however, with magnetom- 

 eters of the Department type Nos. 2-10, should deflection distances less than 25 centimeters be 

 used. Suitable pairs of distances would be 25 and 30, 27.5 and 35, and 30 and 40 cm.; in cases 

 where the deflection angles at distance 40 are so small as to be difficult of detennination with the 

 requisite accuracy, deflections at the first two pairs of distances will be sufficient if suitably distributed 

 through the work. 



Directions of September 7, 1911, for Ocean Observations with Sea Deflector 4. 



1. Follow the general scheme at present in use and as given in "Memoranda regarding sea 

 deflector 4, February 13, 1911," taking 5 readings, in each position, for both sea deflector and Kelvin 

 compass. Two full minutes must be allowed, after magnet is in position, at beginning of observa- 

 tions for each magnet (not distance), as also between each reversal of sights (bowl) ; one full minute 

 must be allowed between all other positions; allowing about one minute for the 5 readings, the 

 minimum time required for a half-set, from beginning of reading to end, Vidll be 8 to 10 minutes, 

 allowing for interruptions and repetitions. 



2. Every possible precaution is to be taken against setting up motion of liquid in bowl by 

 avoiding sharp or rapid reversals of sights. Care must also be taken to avoid possibility of card being 

 lifted off the pivot during reversals of sights by the action of deflecting magnet; the latter, during 

 such reversals, is to be removed and held far enough away, and then replaced. 



3. Time and temperature are to be recorded a few seconds before the beginning of a set, so as to 

 correspond as nearly as possible with actual beginning of set. Endeavor should be made to secure 

 as unifoi-m conditions of temperature as possible during the time the deflecting magnet is above and 

 when below the card. To secure these conditions, the obsei-ving dome should be covered suflaciently 

 to prevent the Sun from shining directly on the magnet or on the card; also the binnacle door is to be 

 kept open throughout the series of observations. 



4. To vary conditions, and to give each magnet the same treatment, begin on alternate days 

 with magnet 45, distance 1, and magnet 2L, distance 1. 



5. It should be kept in mind that the careful work of the deflector-observer may be vitiated by 

 the recorder's poor readings of ship's head \vith the control (Kelvin) compass. There should, 

 therefore, never be any hesitation to repeat observations with deflector and compass, whenever 

 necessary. Readings during rapid motions of compass cards should be avoided as far as possible. 

 The deflector-observer, after making several approximate settings at beginning, should aim to make 

 settings by moving sight-line slightly beyond the 0° or 180° of card, and then working back to desired 

 position; the direction in which this is done should be alternated, once from the right, next time 

 from the left, etc. The sighting slit is to be used preferably, as it affords least opportunity for 

 parallax or "personal equation." 



6. Eveiy possible care must be bestowed on the preservation of constancy of magnetic moments 



of magnets. 



7. Upon conclusion of deflector observations, the bowl will be clamped. 



General Directions of September 7, 1911, for Shore Observations. 



After paj-ing attention to the general remarks contained in directions of August 30, 1910 (see 

 p. 320), according to which the shore work has been done hitherto, the observations may now be 

 confined to the following, two stations being selected as free from local disturbing influences as 

 surroundings permit, and care being taken to have, as far as possible, the same height above ground of 

 magnets of the various instrxmients: 



