General Methods of Work 19 



ing to the methods followed whereby the observer is obliged to make a preliminary 

 reduction before leaving a station, he can himself detect such cases if he has not 

 already done so while observing; he will then, before departure, make another series 

 of observations. 



Intercomparisons of Magnetic Instruments. 



Stations. When the preliminary arrangements have been made, tests are carried 

 out to determine whether, at the proposed site for the intercomparisons, any 

 pronounced local magnetic disturbance exists. If so, and another site for any 

 reason can not be chosen, particular care is exercised to see that, at the same 

 station, the magnetic systems of the various instruments are in the same horizontal 

 plane. In case this procedure is not possible, the height of magnet from a suitable 

 reference point, e. g., from the top of a stake driven into the ground, is noted and 

 determinations are made at each station to find the necessary corrections for the 

 various levels in which the intercomparisons had to be made. Whenever possible, 

 disturbed sites are avoided, but this can not always be done, as for example, in the 

 ocean work where islands often afford the only opportunity for intercomparisons. 

 An entirely undisturbed island is not of frequent occurrence. 



Generally but two stations are required, which, unless already named, as may 

 be the case at observatories, are designated. A, B. For observatory work B is the 

 auxiliary station and A the regular observing pier; at some observatories different 

 piers or stations are used for the various elements and intercomparisons for each 

 particular element must be made accordingly. The azimuth lines for both stations 

 are preferably referred to the same determination of azimuth, in particular, when 

 no exchange of stations can be effected. Where possible both stations are placed 

 in the same azimuth line and the same mark is used at both stations, thus assisting 

 in the avoidance of extraneous error. Triangulation between stations for azimuths 

 of marks is resorted to only when absolutely necessary. In cases where it is not 

 feasible to use the regular observatory piers, a second auxiliary station is established 

 in accordance with the directions already given. 



MetJiod of observations. To secure reliable results expeditiously simultaneous 

 observations with the instruments being compared are preferred, as also an exchange 

 of stations; in this way any possible station difference may be eliminated and the 

 desired instrumental difference be derived without recourse to auxiliary instru- 

 ments, e. g., magnetographs. At observatories where the same piers used in de- 

 termining the magnetograph base lines may be utilized and the required magnet- 

 ogram data be obtained promptly, there may be no necessity for an exchange of 

 stations and simultaneity of observations, though this is found, in general, to be 

 the better procedure. When tripods must be used, each instrument is mounted 

 each time on its own tripod. When, for some reason, simultaneous observations 

 are not possible, the observations are carried out alternately at each station by 

 the same observer, thus, with the two instruments 1 and 2, and the stations A 

 and B: observations with 1 at ^, with 2 at B; 2aXB,\ at A; I at A, 2 at B; 2 at B, 

 1 at ^; and so on ; next, 2 at ^, 1 atB;l at B, 2 at A; 2 at A, 1 at B; 1 at 5, 2 at A; 



