212 Reports on Special Researches 



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 deter- 

 mination of azimuth, especially when no exchange of stations can be effected. Whenever 

 possible both stations are placed in the same azimuth line and the same mark is used at each, 

 thus assisting in the avoidance of extraneous error. Triangulation between stations for 

 azimuths of marks is resorted to only when absolutely necessary. 



To secure reliable results expeditiously, simultaneous observations with the instruments 

 being compared are preferred, as also an exchange of stations; in tliis way any possible station- 

 difference may be at once eliininated and the desired instrument-difference be derived without 

 recourse to auxiliary instruments, e. g., magyietographs. At observatories where the same piers 

 used in determining 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 with the two instruments 



1 and 2, and the stations A and B as follows: observations with 1 at ^, with 2 at B; 



2 at 5, 1 at A; 1 at ^, 2 at B; 2 at B, 1 at A; and so on; next, 2 at ^, 1 at B; \ &i B,2 at A; 

 2 at A, 1 at 5; 1 at B, 2 at ^; and so on. As little time as possible is allowed between 

 determinations at the two stations in order to minimize outstanding effects of corrections 

 to common epoch. With the number of determinations called for, this scheme of observa- 

 tion, while of course not as good as simultaneous intercomparisons, nevertheless yields 

 good results when used with care. 



Whenever possible, the practice is to secure with each instrument at least 12 complete 

 determinations of declination, 6 at each station; 6 complete determinations of horizontal 

 intensity, 3 at each station (one determination consisting of two sets of oscillations and two 

 sets of deflections at two or more distances) ; and at least 6 determinations of dip with each 

 needle, 3 at each station. The observations are made for different orientations of the foot- 

 screws of the instruments, preferably so that there will be an equal number of observations 

 at each station for footscrew marked A south, footscrew B south, and footscrew C south. 

 The work for any one element is not completed on one day, but distributed over several 

 days in order to minimize a possible effect dvie to magnetic perturbations. Where an exchange 

 of stations is not practicable, the total number of determinations for each element is at 

 least as great as just stated. Particular care is used to see that the instruments are in good 

 working order and the requisite caution is exercised to insure the absence of disturbing 

 influences of whatever character. Before leaving the station, the computations are com- 

 pleted far enough to make sure, at least, that no observational blunders have been made. 



Of special interest will be found the tables exhibiting the results of comparisons 

 secured at the same observatory at diff'erent times, e. g., before and after strenuous field 

 work, or by different observers of the Department with different instruments. These 

 repetitions will serve as some guide in answering the questions raised above, and in show- 

 ing how closely the Department of Terrestrial Magnetism, with its instruments and its 

 methods, can reproduce its standards in remote parts of the Earth. 



It is a pleasure to record our indebtedness to the directors of the various observatories, 

 and to the members of their staffs, for the very cordial assistance rendered, as well as our 

 appreciation of the uniform courtesies extended to the representatives of the Department. 



The instruments used by the Department observers are designated by their respective 

 numbers, which will serve at the same time, by referring to Vol. I (j^p. 2-11) and the 



