TERRESTRIAL MAGNETISM IN THE TWENTIETH 



CENTURY' 



By Daniel L. Hazard 

 Coast and Geodetic Survey 



One of the early presidents of the American Association for the 

 Advancement of Science in his presidential address likened himself 

 to a biennial plant, which for the first year devotes itself to storing 

 up a reserve supply of plant food and in the second j^ear bursts into 

 flower. The character of the flower is an indication of the adequacy 

 of the stored food supply. The president of this society is in much 

 the same category. 



The close of the nineteenth century witnessed a marked quickening 

 of interest in the study of the earth's magnetism, of which one evi- 

 dence was the expansion of the magnetic work of the United States 

 Coast and Geodetic Survey, the establishment of a separate division 

 of terrestrial magnetism, and the inauguration of a magnetic survey 

 of the United States. 



Now, after the lapse of a quarter of a century, it will not be amiss 

 to take account of stock and see to what extent our knowledge has 

 been increased and how far we have advanced toward the solution of 

 the perplexing problems of the causes of the earth's magnetism and 

 its variations. 



It must be borne in mind that the science of terrestrial magnetism 

 is comparatively young. Although Gilbert in 1600 conceived the 

 idea of the earth as a great magnet, similar to a spherical lodestone, 

 he had almost no observational data with which to test his theory, 

 and it was not until 1838 that a fairly correct idea of the nature of 

 the earth's magnetism and its distribution was developed by Gauss 

 from his analysis of the results at that time available. With the 

 character of the phenomenon established, the natural philosophers 

 were quick to realize the importance of observations more widely 

 distributed over the earth as a prerequisite for a more effective study. 



1 Address of tlie retiring president of the rhilosophical Society of Wasiiington, presented 

 at the meeting on Jan. 10, 1925. Reprinted by permission from tlie Journal of the 

 Washington Academy of Sciences, vol. 15, No. 6, Mar. 19, 1925. 



243 



