124 Ocean Magnetic Observations, 1905-16 



b. To afford some experience with the Kelvin compass, the latter should be mounted when 

 no horizontal-intensity observations are being made with the sea deflector, and dechnations 

 should be observed with it. In order to determine the ship deviations for the Kelvin compass, 

 as also those for the Negus compass when used for declination work, several comparative readings 

 of the lubber-lines with the Ritchie standard (RIB) should be made before and after a series of 

 declination observations on any particular course. 



c. When the Kelvin compass is mounted, attempt should be made to determine the period 

 of vibration of the particular card used, and a record be kept of the various places at which these 

 experimental observations are made. (The purpose is to ascertain whether a vibration method 

 could be as advantageously employed as the present deflection method. The Kelvin compass 

 will of course not be kept on the bridge when likely to affect the other work going on at the time. 



d. Occasional horizontal-intensity observations with the sea-deflector attachment mounted 

 on the Ritchie standard compass (RIB). 



8. Meteorological observations. — These observations will be made in accordance with the direc- 

 tions suppUed by the United States Weather Bureau. Whatever additional marine observations 

 may be possible must be left to the commander's discretion. 



9. Reduction of observations. — The endeavor should be, in general, to keep computations up to 

 date, though they should not be allowed to interfere with the time for observations. No attempt 

 is to be made to determine and apply precise corrections, the scheme of work in fact being now 

 arranged so as to avoid the necessity of applying such corrections, as, e. g., position corrections. All 

 corrections to be appHed, whether for position or for reduction to standard, now solely affect the A 

 coefficient of any element involved, and will not affect the harmonic coefficients B, C, D, and E. 

 The analysis will invariably be made without A, and the final determination of A left to the Office. 

 It will be best accordingly, at present, not to apply any corrections of the nature mentioned. 



10. Records of observations. — The original records are to be promptly forwarded, by registered 

 mail, at each port of caU, and not be allowed to accumulate any more than necessary. Even if the 

 computations are not complete, it will be preferable to transmit the records. Abstracts should be 

 made of the essential quantities to guard against loss of work in transmission to the Office, and for 

 use aboard ship. There will thus be afforded the Office opportunity to determine whether any 

 revisions in directions are necessary. 



11. Shore observations. — Descriptions of stations and directions are embodied in a separate 

 communication. 



From Instructions of February 10, 1906, Regarding Land Magnetic Observations, 



TO W. J. Peters. 



1. The land observations will in general be made principally with the Coast and Geodetic Survey 

 magnetometer 36, sea dip-circle 35, land dip-circle 178, and sea deflector 1 mounted on Negus com- 

 pass, using magnets 45 and NL. The observations should be arranged so as to vary the corrections 

 for diurnal variation and to reduce the effect from possible magnetic storms as much as possible by 

 distributing the observations over two or more days (if there be opportunity), rather than increasing 

 the number on one day. They are also to be arranged so as to secure as good comparisons as pos- 

 sible of the instruments. The results are to be completely reduced before the vessel leaves, in order 

 to afford opportunity for repetition, or to ascertain the cause of any observed discrepancy in the 

 constants or results. The importance of the land work in the determination of the final corrections 

 and constants for the ship work must not be overlooked. 



2. When the Negus compass is mounted on land, declination observations should be made, 

 preferably both by direct reading of azimuth mark and by sun-azimuths, so as to test the azimuth 

 devices used on board ship. Adjustments should not be disturbed, however, unless there be good 

 reasons; in the latter case, full record must be made. The same statement applies to the testing of 

 the Ritchie standard compass and azimuth arrangements, and when determining the intensity con- 

 stant for the azimuth circle used in connection with any deflector observations that may have been 

 made. So Ukewise, if the Kelvin compass has been used for occasionally getting vibrations and 

 dechnations, certain shore observations will be desirable. These matters must be largely left to the 

 commander's discretion, as they depend upon the precise circumstances involved. Shore observa- 

 tions and intercomparisons or tests of instruments may at times be facilitated by establishing a 



