274 
LITTLEHALES. 
the case of the declination, except in the long-settled regions 
of European civilization. This is accounted for by the fact 
that the dip was rarely observed by navigators, except when 
employed in expeditions of scientific research, while the dec¬ 
lination was found as a necessary performance in the navi¬ 
gation of their ships. The investigation of the long series 
has led to the belief that the secular variation of the inclina¬ 
tion is also a periodic phenomenon. But the data which have 
been observed up to the present are manifestly insufficient 
to warrant a conclusion that after a certain period has elapsed 
the declination at any given station will be the same as it is 
now, and will then repeat its changes and again assume the 
same value after the lapse of the same interval of time, or 
that the inclination at that place will be found to pass 
through a cycle of changes and return to the same value at 
regular intervals of time. While the separate investigation 
of series of observations of declination and inclination is of 
practical usefulness in gaining a knowledge of the rate of 
secular change of these elements and predicting values be¬ 
yond the range of the observations, in seeking to discover 
the causes of the secular change in the direction of the mag¬ 
netic needle and to establish or disprove its periodic char¬ 
acter, the declination and inclination should be viewed as 
component effects of the forces that are acting. Such a view 
brings us to the investigation of the successive directions in 
space assumed at successive epochs by a freely suspended 
magnet or the consideration of the observed values of the 
declination and inclination conjointly, instead of the sepa¬ 
rate consideration of values of the direction of the compass 
needle and of the dipping needle. As a freely suspended 
magnet assumes its successive directions for different times, 
it describes a conical surface whose vertex is the center of 
gravity of the needle. 
If a sphere of any convenient radius be described, with its 
center coinciding with the center of gravity of the needle, and 
the conical surface be extended through the surface of the 
