122 Ocean Magnetic Observations, 1905-16 



Directions of February 8, 1906, for Swings No. 3 at San Diego, to W. J. Peters. 



Same as for swings No. 2, except for following modifications : 



1. First hehn: Deflections with sea dip-circle, beginning with long distance, instead of short 

 distance; face of needle 3 to be towards observer for long distance and away from observer for short 

 distance. 



2. Other helm: Loaded dip and regular dip (needle No. 2 again, but this time B end down) with 

 sea dip-circle. 



3. First hehn : Deflections with sea deflector, using this time magnet NL with letters up through- 

 out. 



4. Other helm: Deflections with sea deflector, again using magnet NL, but now with letters 

 down throughout. 



5. Declinations to be obtained by the observer at the Ritchie standard compass (RIB), taking 

 3 readings, on each heading, for both hehns. 



Instructions of February 9, 1906, for Sea Observations, to W. J. Peters. 



1. When all instructions regarding the observations and computations for the work at San Diego 

 have been completed, and the original records have been forwarded to the Office, please wire when 

 you are ready to go to sea, and await telegraphic advice. You have already received instructions 

 regarding route (Jan. 20, 1906). 



2. Disturbing Causes. — You have already been cautioned as to the need of excluding, so far as 

 lies within your power, any extraneous, artificial disturbing influences hkely to affect the work 

 assigned. You will accordingly see to the proper disposition of articles on and in the vicinity of the 

 bridge, removal of magnetic articles from the observer's person, etc. Each observer will state on his 

 record sheet of observations whether he had removed all magnetic substances. Statements regarding 

 the various conditions under which observations and swings are made can not be too full. Roll of 

 vessel, weather conditions, condition of ship, manner of swinging, and all other pertinent facts should 

 receive proper attention in the notes. 



3. Geographic position of ship during ohservatio7is. — The need of giving this matter your special 

 attention has already been fully explained. The observations and computations should be arranged, 

 as far as possible, to afford opportunity for independent checks. 



4. Swings at sea. — A complete swing at sea must be obtained with both helms, covering 8 points 

 as well as conditions permit, as soon as possible after leaving San Diego Harbor. Thereafter the 

 attempt mil be made to obtain a swing every third day, and, in no instance, should more than a week 

 be allowed to elapse between two swings, if conditions of sea and weather do not prevent. Whenever 

 possible the three magnetic elements, declination, inclination, and intensity, are to be secured on the 

 same swing. Cloudiness, however, will at times prevent this ideal combination and the work will 

 have to be arranged accordingly. Cloudiness will not be an excuse, however, for not swinging at 

 least for inclination and intensity; the ship deviations for both of these elements, on account of their 

 greater comparative magnitude and greater susceptibihty to change than the decUnation deviations 

 at the position of the Ritchie standard (RIB), require more frequent control. 



The general program of magnetic work for a swing will be as follows : 



a. Dechnation observations with the Ritchie standard compass (RIB), securing 3 readings 

 so far as possible on each heading of both port-helm and starboard-helm swing, increasmg the 

 niunber, if necessary, in accordance with the conditions encountered. 



b. Horizontal-intensity observations with sea deflector (Dl).— Magnet 45 is to be used 

 throughout for both helms, having letters "up" for first swing, and letters "down" for return 

 swing. Invariably on each heading the four positions or readings will be taken. 



c. Inchnation and total intensity with sea dip-circle (D. C. 35).— First helm: Loaded dip 

 (needle 4, weight 6) , invariably on each heading for the two positions circle east face of needle 

 east and circle west face of needle west, needle not to be turned around (inverted) throughout 

 the swing, and at least 2 independent readings to be secured for each end of the needle in each of 

 the two positions prescribed. Other helm : Deflections, using bo th distances.^ Thus, e.g., short 



' Both deflection distances may require too much work. If so, it will suffice to take but one distance, using for one 

 station the short one, for the next station the long one, thus alternating the two distances. When a single distance is used, 

 then 2 readings should be taken on each end of the needle. Whenever but one end of a needle can be used, whether in dip or 

 intensity work, then readings on the visible end will be multiplied. 



