PROGRESS IN SCIENCE IN TERRESTRIAL MAGNETISM. 333 
variations in the direction of the magnetic needle and the 
intensity of the Earth’s magnetic force through which ter¬ 
restrial magnetism has been interrelated with meteorology, 
geology, and solar physics, and who gave the mathematical 
theory of terrestrial magnetism which has shaped the prog¬ 
ress of all subsequent investigations. His analytical ex¬ 
pressions for the distribution of the terrestrial magnetic 
forces represent the highest stage of completeness attainable 
with the material available in his time. He greatly re¬ 
gretted that the inaccuracy of some of the data and their in¬ 
sufficiency did not permit of a more accurate determination 
of his coefficients. Improvement could only be hoped for 
with the addition of new material and so his first endeavors, 
in which he was ably supported by Weber and Lamont, were 
to obtain new data and improve the instruments and meth¬ 
ods of observation. His method of deriving an analytical 
expression to represent the facts of the Earth’s magnetic 
state as manifested at its surface was by expressing its poten¬ 
tial in a series of solid harmonics, the coefficients of which he 
determined for the first four degrees in order to secure a 
tolerably accurate determination of the actual state of the 
Earth’s magnetism. These methods imply that a complete 
magnetic survey of the Earth’s surface has been made and 
that the values of the terrestrial magnetic components are 
known at a given epoch. There have been, however, at all 
times in the past, and there are at present, large tracts of the 
Earth’s surface about which we have no data, so that the full 
benefit of the classical investigations of Gauss can not yet 
be reaped. 
It is not alone the distribution of the Earth’s magnetism 
that has received investigation, but its variations with time 
have also been made a subject of profound study. By 
comparison of the early charts of the isomagnetic lines of 
the globe with those of the present epoch, we are struck 
with the changes which time works in the Earth’s magnetic 
state and are brought to a realization of the fact that we are 
still unacquainted with one of the powerful forces in ter¬ 
restrial physics. 
