GENERAL METHODS OF WORK. 

 Selection and Description of Station. 



The conditions to be satisfied in choosing a magnetic station are freedom from 

 present and possible future local disturbances, and, as far as possible, convenience 

 of access. Selections must be such that no electric railways, masses of iron or steel, 

 buildings of stone or brick, or other known artificial sources of disturbance are near; 

 in general it has been found that at least 1 kilometer from electric lines and about 

 0.2 kilometer from the other disturbing influences mentioned are safe distances 

 for the usual field-work. When there is any doubt as to the existence of local 

 disturbances, two intervisible points, 300 meters or more apart, are selected and 

 the magnetic bearing of the line joining them is observed from both points; this 

 may be done with the transit compass or with the dip-circle compass. If there is 

 indication of local disturbance, another location is made and a test again applied, 

 etc. When possible, stations are located on public property, or property belonging 

 to an educational institution, because of greater probability of permanence of such 

 locations. 



In the case of old stations special effort is made to reoccupy the precise point 

 if the local conditions permit; and if not, a new station is established. Sometimes, 

 however, not only is a new station selected, but the old one also reoccupied; the old 

 station, while not satisfying the requirements of future availability, may still 

 suffice for determining the secular variation; in such a case sufiicient observations 

 are made to effect a good transfer from the old to the new site. Secondary or 

 auxiliary stations, such as are required, for example, in the intercomparison work, 

 are, in general, located on a line of known azimuth from the primary station. 



Every station occupied is described, whenever possible, with sufficient detail 

 to permit its ready recovery. Photographic views are taken whenever possible 

 and a sketch is made of the station with its surrounding objects, and measured 

 distances to nearby objects are given. Particular care is exercised in the spelling 

 of geographical names, especially so with regard to names of places in comparatively 

 unexplored countries, where much of the work of the Department must be done; 

 the adopted spelling is usually that most commonly used locally. 



The station is marked in as permanent a manner as local conditions permit. 

 The most desirable form of marking is by means of a non-magnetic stone monu- 

 ment, a drill hole in the top, 1 or 2 cm. deep, serving to indicate the precise point; 

 these markers are set sufficiently deep in the ground to be free from frost effects, 

 as also to prevent ready removal by unauthorized persons. Where convenient, 



the top of the marker is lettered or numbered, for example, thus: 



In case a stone post can not be procured, a wood post set firmly in the ground may 

 be used or a mark may be cut in a surface stone. In general, however, only such 

 stations as give promise of permanency are marked in some durable manner. 



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