126 Ocean Magnetic Observations, 1905-16 



3. The liability to changes in ship deviations dependent upon length of time on course pursued 

 or on direction of vessel while in harbor, or upon conditions of sea and weather, etc., should be borne 

 in mind. Hence swings should be secured at sea as soon as possible after leaving, or before entering 

 a harbor, and the harbor swing be made immediately upon arrival. 



4. The San Diego observations should be mailed before departure. 



5. The sea swings will be obtained as often as conditions of sea, weather, and time permit. 



6. Satisfactory swings in one port at least (if possible two ports) in the Southern Magnetic 

 Hemisphere are required. 



Directions of November 30, 1906, for Course Observations, to W. J. Peters, San Diego. 



1. The course observations will in general be made as on previous cruise, endeavoring, if 

 possible, to obtain the three magnetic elements (declination, dip, and intensity) for the same 

 geographical position. 



A. Declinations will be obtained as frequently as possible with both Ritchie compass and 

 Negus compass, laying principal stress, however, upon the former instrument. Declinations 

 with the Kelvin compass are optional ; it will not be amiss, however, to experiment further with 

 the various azimuth devices employed with this instrument. As indicated under "Swing direc- 

 tions" (ID), it is especially desired to try this compass for vibration observations. (If such 

 experiments have been made, corresponding shore observations are required for determination 

 of intensity constant.) 



B. The sea-deflector work, simultaneous with the sea-dip-circle observations, will consist 

 of horizontal-intensity observations, using both magnets 45 and NL (only short distance in each 

 case), letters up and down, same as hitherto. (When observations are possible with only one 

 magnet, use 45). 



C. The sea-dip-circle work will consist, as heretofore, of total-intensity observations made 

 between the regular-dip observations. Only one deflecting distance will be used throughout, 

 observing, however, each time with both face "direct" and "reversed" of suspended needle. 

 For the region in which the dip is above 40°, alternate long and short distance will be used, 

 whereas for region below 40° only long distance. [Supplemented on Dec. 10, 1906, as follows: 

 Alternate dip circles 35 and 169, i.e.,\i 35 was used at one place of observation, then use No. 169 

 the next time. In case of No. 169, however, observe deflections with both distances, each time 

 whenever possible. For symmetry of the work, it would therefore be desirable to make a double 

 set of deflections with No. 35 for the single distance used.] 



D. Miscellaneous observations, astronomical, meteorological, etc., to be made as oppor- 

 tunity affords. 



2. While it will be best that each observer have his own particular instriunent throughout the 

 cruise, it is desired, however, that each one familiarize himself sufficiently with the instruments and 

 work of the others, so that, if suddenly called upon, he may be able to perform another's duties. 

 Observers must bear in mind that for successful office reductions notes can not be too full. 



Directions of November 30, 1906, for Land Observations, to W. J. Peters, San Diego. 



1. Besides the observations with magnetometer and land dip-circle, dips and total-intensity 

 observations are required, in each instance, with the sea dip-circle. Deflections, both distances, will 

 be made at San Diego, Tahiti, Zikawei, Honolulu, Dutch Harbor, Sitka, and again at San Diego; 

 at the other ports only long distance will be used. [Supplemented on December 10, 1906, as follows : 

 On land, both sea dip-circles 35 and 169 will be used. Where in the case of No. 35 only one deflection 

 distance is prescribed, invariably make a double set. With No. 169, deflections will always be made 

 with both distances until otherwise instructed. With No. 35 both deflection distances are to be 

 used at San Diego, Tahiti, Zikawei, Honolulu, Dutch Harbor, Sitka, and San Diego, whereas with 

 magnetometer 1 both deflection distances are to be used at Tokio, Honolulu, Dutch Harbor, Sitka, 

 and San Diego.] 



2. Make necessary shore observations (lubber-line on 8 points) for testing compasses and azimuth 

 devices used in declination work. (When theodolite-azimuth method is used, as on previous cruise, 

 all time data required for likewise computing azimuths should be given without fail.) 



