330 
LITTLEHALES. 
travel toward the southern magnetic pole similar changes 
in inclination take place with reference to the south-seeking 
end of the needle. Analogous results may be obtained by 
carrying a small needle through the magnetic field of a bar 
magnet. At the neutral band, it will be parallel to the bar, 
while as either end is approached, the inclination toward the 
pole becomes greater and greater; and as with the bar mag¬ 
net, so with the earth, there is a magnetic field varying in 
intensity of force from point to point. There is thus a third 
set of lines passing through all points where the intensity 
of the Earth’s magnetic force is the same. These are known 
as lines of equal magnetic intensity or isodynamic lines. 
In general contour they follow the lines of equal inclination 
or dip. 
These different systems of lines representing the values of 
the magnetic elements have not on the earth that symme¬ 
try and regularity which they would present around a 
steel bar, but, on the contrary, they often pursue serpentine 
courses, and since the values of the magnetic elements are 
not fixed either as to time or locality, they shift their posi¬ 
tions hourly, daily, monthly, yearly, and through centuries. 
What Halley did in the beginning of the eighteenth cen¬ 
tury was to gather together the observations of the variation 
of the compass or the magnetic declination that had accu¬ 
mulated in England and publish them in a world-chart 
showing the lines of equal variation of the compass, for the 
year 1700, in all the seas excepting the Pacific Ocean. In 
describing this chart the author wrote: “What is here prop¬ 
erly new is the curve lines drawn over the several Seas, to 
show the degrees of the variation of the magnetical 
needle or sea-compass: Which are designed according to 
what I myself found, in the Western and Southern oceans, 
in a voyage I purposely made at the Public charge, in the 
year of our Lord 1700; or have Collected from the Compari¬ 
son of several Journals of Voyages lately made in the 
Indian Seas, adapted to the same year.” 
Two-thirds of a century after the appearance of the first 
