W. A. Norton on the Variations of the Declination, §c. 183° 
Arr. XIX.—On the Periodical Variations of the Declination and 
Directive Force of the Magnetic Needle; by W. A. Norton, 
Professor of Civil Engineering in Yale College. 
throughout the greater part of the northern hemisphere the north 
end of a needle thus suspended is depressed below the plane of the 
horizon ; and that throughout the greater part of the southern hem- 
isphere the south end is depressed below this plane ; also that this 
inclination or dip of the needle gradually increases from the mag- 
netic equator, where it is zero, in both directions to the magnetic 
poles, where it is 90°. Such then are the varying directions of the 
directive force of the magnetic needle at different points of the 
earth’s surface. The two ends or poles of the needle are solicited 
IM opposite directions ; the north end downward, and the south 
end upward. It suffices in discussing the perturbations to which 
the earth’s magnetic force is subject, whether of direction of in- 
tensity, to confine our attention to the action upon one end of the 
needle, for example the depressed end,—(north end in the north- 
Sorce, and the vertical force, acting upon the needle ; or the hori- 
zontal and vertical magnetic intensities of the place. These two 
forces, together with the declination (or deviation of the line of 
direction taken up by the horizontal compass needle, from the true 
north and south line) constitute what are called the Magnetic E'le- 
ments of the station. Each of these elements changes in value as 
We pass from one station to another. It is well known also that at 
any one station they are not at all times unalterably the same ; 
but are subject to variation from hour to hour, from day to day, and 
from year to year. The changes from hour to hour during the 
. 
tions 
ton to the term annual variation, of a magnetic element. But, 
€ present paper I shall generally use these terms in the most 
