METEOROLOGY. 255 
in the ocean, and is therefore readily accessible. Those places on the 
earth where the dipping needle stands vertically are called the magnetic 
poles; of these there are two, one in the southern and one in the northern 
hemisphere. They are also characterized by the fact, that in all regions 
near one of these poles the horizontal needle is directed immediately 
towards it. PJ. 28, figs. 1* and 1°, represent the isoclinic lines of the 
northern and southern hemisphere for the year 1825, after the chart of 
Admiral Duperrey, who crossed the magnetic equator six times between 
1822 and 1825. 
Isodynamic lines connect those places which possess the same intensity 
of magnetism. Fig. 2 exhibits these lines for two hemispheres, likewise 
after Admiral Duperrey. In general the magnetic intensity increases from 
the terrestrial equator to either pole, but the isodynamic lines run parallel 
neither with the magnetic nor the terrestrial equator. An arbitrary unit 
has been made of the intensity observed at the magnetic equator in Peru 
by Humboldt, although this is by no means the minimum of observed 
intensity: this real minimum amounts to seven tenths of that assumed as 
the standard, and occurs on the coast of Brazil. By connecting those 
places in each meridian in which the intensity attains its minimum, we 
shall have a line, called by Duperrey the magnetic equator, which, however, 
by no means appears to coincide with the aclinic line, or line of no 
inclination. The maximum of known intensity amounts to a little over 
two, and occurs near the south magnetic pole; the observed maximum is 
thus about twice the minimum. Near the magnetic north pole in 
Melville Island it only amounts to 1.6. (See pl. 28, figs. 1% and 1’, where 
the term “magnetischer (magnetic) equator,” is to be taken in the meaning 
of Admiral Duperrey.) 
The oscillations of the magnetic needles depend much on the course of 
the sun, so that at one and the same place the time of day may be 
ascertained by the position of the needle. The hourly variations of 
declination increase in extent with the magnetic latitude or the distance 
from the magnetic equator; thus, in Middle Europe they amount to 
thirteen or fourteen minutes, and near the equator to but three or four 
minutes. Jn the whole northern hemisphere the north end of the needle 
appears on an average to move westwardly from 83 A.M. to 13 P.M., and 
eastwardly in the same interval in the southern hemisphere. It is quite 
probable that between the geographic and magnetic equators there is a 
region where no hourly variation of declination exists ; this curve, however, 
has not yet been discovered. Sometimes there occur extraordinary 
disturbances or perturbations of the needle (magnetic storms), which are 
propagated in immeasurably short spaces of time in every direction over 
the earth’s surface, being perceived at the same instant at the most distant 
stations. For this reason they may be employed in determining the 
geographic longitude. 
The aurora exerts a greatly disturbing influence on the magnetic needle, 
for which reason this remarkable phenomenon is now almost universally 
considered as magnetic in its character, although no satisfactory explanation 
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