272 
LITTLEHALES. 
self* to portray the secular course of a freely suspended 
magnet, and thus, by providing homogeneous data concern¬ 
ing all parts of the world, to contribute toward an advance 
of our information to such a stage as to make the study of 
the causes of the secular change in the earth’s magnetic 
state a feasible undertaking. 
A freely suspended magnet is observed to be in a state 
of continuous tremulous motion of an involved character, 
which may be resolved into irregular and periodic. The 
irregular motions comprise those sudden and rapid fluc¬ 
tuations in the direction of the needle which can not be 
predicted. The periodic motions are the solar variations, 
which include the solar-diurnal variation, depending upon 
the hour of the day ; the annual variation, depending upon 
the day of the year, and the solar-synodic variation, depend¬ 
ing upon the synodic revolution of the sun ; the lunar vari¬ 
ations, depending upon the moon’s hour-angle and her other 
elements of position and partaking of the character of the 
tides, and the decennial variations, which may depend upon 
the frequency and magnitude of the solar spots. Both the 
irregular and periodic motions referred to are of such small 
amplitude in all except the polar regions of the earth that 
they do not affect any of the practical uses of the magnetic 
needle on the sea ; but besides these there is another motion, 
having an amplitude reaching 30° or 40° in some parts of 
the world, which is also supposed to be of periodic character, 
and which is doubtless of radical importance in meteorolog¬ 
ical science and in the use of the compass in modern nav¬ 
igation. 
At a particular instant of time the lines of magnetic force 
at any place to which a freely suspended magnet will set 
itself tangent will have a certain direction and strength. 
The angle between the plane of the astronomical meridian 
* (a) Contributions to Terrestrial Magnetism. U. S. Hydrographic 
Office Publication No. 109a, 1895. 
(6) Contributions to Terrestrial Magnetism. U. S. Hydrographic Of¬ 
fice Publication No. 114, 1897. 
(c) Terrestrial Magnetism, vol. I, No. 2, 1896. 
