DEPARTMENT OF TERRESTRIAL MAGNETISM. 253 



and I and c = thickness of intervening lenses. A close study of Kohlrausch 

 would seem to indicate the correct formula to be 



j?-n ^ ^ ^ 

 The experiments confirmed the correctness of the modified formula. 



The Bermuda magnetic anomaly and possible effect on secular changes.^ H. W. Fisk. 



The peculiar interest in the local magnetic disturbances in Bermuda arises 

 from the geological condition with which it is connected. The island is a 

 submerged mountain of volcanic origin covered with a coral-limestone cap 

 which, at the only point where a boring has been made, extends 250 feet below 

 sea-level. 



The author, under the auspices of the Department of Terrestrial Mag- 

 netism, made a magnetic survey of the island in 1907 and repeated a part 

 of the work in the summer of 1922, carrying out at the same time some special 

 investigations relating to local disturbances. In this later work, extensive 

 tests were made which failed to discover any magnetic properties in the lime- 

 stone, either in fragments or in large masses. The magnetic distribution, 

 however, indicates that, besides the general disturbance affecting widely 

 separated areas, there are others of local character that have their origin 

 much nearer the surface. The compass-variometer, an instrument designed 

 and constructed by the Department for the detection of small differences of 

 horizontal intensity, was used to study a few limited areas in detail. In 

 one such there were found differences of as much as 700 gammas (0.00700 

 c. g. s.) within less than 20 feet horizontal distance. There was no evidence 

 of an artificial source for this disturbance. The soil at various points on 

 the island was examined and found to be magnetic, though the distribution 

 of these local disturbances could not be directly related to the depth or other 

 apparent properties of the soil. There are, therefore, a major or primary 

 source of disturbance lying deep in the lower structure of the submerged 

 mountain, and other minor or secondary sources lying in or near the surface, 

 probably in the soil. 



Diurnal-variation observations with field instruments were made in 

 declination simultaneously by two observers stationed at two points between 

 which the differences in the magnetic elements were near the maximum 

 amount. The simultaneous declination differences remained practically 

 constant, and no evidence of any change in the diurnal variation either in 

 range or phase due to the existence of the disturbance was found. A similar 

 series of observations was made for the inclination, using earth inductors, 

 obtaining a value every 15 or 20 minutes, and for horizontal intensity using 

 the method of deflections at one distance repeated at similar intervals. For 

 these elements good diurnal-variation curves were obtained, although, because 

 of the magnitude of unavoidable errors of observation, they were not a 

 satisfactory basis for any conclusions. 



In the course of the work in 1907, five stations were chosen as repeat 

 stations and carefully marked for reoccupation. Two of these were reoc- 

 cupied in 1910 by the Carnegie party, and all were reoccupied in 1922. The 

 small differences in the 15-year change in inclination and horizontal intensity 

 at the different stations show a slight tendency to vary with the value of the 

 element. When the results from the nearest magnetic observatories are 

 available, by which an approximate reduction of observations at all stations 

 to a common epoch may be made, it may be possible to determine whether 



* Based on paper presented before the Section of Terrestrial Magnetism and Electricity of 

 the American Geophysical Union, Washington, April 18, 1923. 



