14 Land Magnetic Observations, 1905-10 



Astronomical Work. 



The observations at each station are of two classes, astronomical and magnetic; 

 from the first are obtained the true azimuths of marks used in the magnetic work, 

 correction of chronometers on local mean time and the geographical position; the 

 second embraces the determination of the three magnetic elements: declination, 

 dip, and intensity. In order to secure the necessary stability, the tripods on which 

 the instruments are mounted are made to rest upon three oak brass-bound stubs, 

 about 20 cm. long, driven well into the ground. 



In general, observations for latitude, time, and azimuth are made preferably on 

 the Sun; only occasionally are they made on stars and planets, night work being 

 usually attended with various inconveniences. 



The theodolite is used either as an attachment or a part of the magnetometer, 

 as in the so-called theodolite-magnetometer and in the universal magnetometer, 

 or is an entirely separate instrument. On the diaphragm of the telescope is ruled 

 a single cross formed of one vertical line and one horizontal line. The graduated 

 horizontal and vertical circles are in general about 10 or 12 cm. in diameter, and 

 the least count of the verniers is from 20 to 60 seconds of arc. The instruments 

 are carefully constructed and the degree of accuracy aimed at in their design is of 

 the order of 30 seconds or less. For the purpose of carrying time, the observer is 

 usually provided with a half-second pocket chronometer and two or three watches, 

 these timepieces being intercompared daily and handled with every care. 



Chronometer record. As often as possible the chronometer and watch cor- 

 rections on standard time are obtained by means of previously arranged telegraphic 

 time-signals. Corrections to standard meridian time are also obtained from the 

 alt-azimuth observations or from special time observations at stations of known 

 longitude. Advantage is likewise taken of opportunities to secure comparisons 

 with ship chronometers, a statement being made as to the reliability of the time 

 furnished. Determinations of chronometer or watch rates are made whenever 

 opportunity permits repeated time observations at the same station or when a 

 previously occupied station, in the course of travel, is again reached; such determi- 

 nations, with an interval of not less than three or four days, are always of value. 

 Daily intercom parisons between chronometers and watches (referring all to same 

 chronometer time) are made and record is kept with pertinent notes as to con- 

 ditions of travel, remarks as to winding, failure to wind, etc. The comparisons 

 are made in general twice dail}^ including intervals of rest or delay as well as when 

 traveling and when at stations; so far as is possible, these intercomparisons are 

 made, from day to day, at approximately the same chronometer time morning and 

 evening. These records and comparisons, as well as signals, are kept according to 

 civil reckoning, counting hours from zero (midnight) through 24 hours. In con- 

 nection with signals or chronometer comparisons, the records kept by the observer 

 show all necessary particulars. 



By "correction to standard" is meant the quantity which must be applied to 

 chronometer or watch time to get the standard meridian time; the correction is 

 thus always positive when the timepiece is slow and negative when fast, and 



