202 PHYSICS. 
Finally, fig. 26 represents the electrical light apparatus, and in fact 
exhibits the apex of the electrometer. a@ is a lantern at the upper end, 
whose lamp burns constantly, b is a copper tube on which tke lantern slides, 
fastened to the glass cone, c; this latter is hollowed beneath and coated with 
copper, under which stands the lamp marked e, constantly burning to heat 
the copper, and thus to maintain the glass cone in a proper condition of 
dryness; d is a wooden apparatus receiving the lower part of the cone, f is 
a conducting wire leading to the electrical apparatus in the antechamber, 
g, g are iron wires, by means of which the whole apparatus may be moved 
up and down. 
The different values of the observed magnetic declination, inclination, and 
intensity, in different parts of the earth, are represented on charts by 
three systems of lines, called after Humboldt, tsogonic, isoclinic, and 
isodynamic. Isogonic lines connect those parts of the earth possessing 
equal declinations. Charts on which they are delineated are called declina- 
tion maps; they can, however, be relied on as accurate for a short time 
only, as the magnetic declination of a place is constantly changing. Never- 
theless there are places on the earth where the declination does not sensibly 
change for a considerable period of time ; among these are Spitzbergen and 
the western part of the Antilles. Among all the isogonic lines the line of 
no deviation is most remarkable, the line connecting all places where the 
needle points due north, or where the magnetic and astronomical meridians 
coincide. This passed through London in 1657, and through Paris in 1669. 
It divides the earth into two portions, of which the one has an eastern and 
the other a western deviation of the needle. The latter portion embraces 
all Europe, excepting a small part of Russia, Africa, and nearly all of the 
Atlantic Ocean. In north-eastern Asia the isogonic lines form a closed system 
of oval outline, this same condition being represented more regularly and of 
greater extent in the South Sea, between 20° N. and 45°S. latitude. In 
the Asiatic oval the deviation increases from without to within, while in that 
of the South Sea the reverse occurs. The chart occupying the middle of 
pl. 28 represents the observed values of the declination in the years 
1827-30, in isogonic lines after Adolphe Erman. The figures accompanying 
the lines indicate the degree of declination, the western being taken as 
positive, the eastern as negative. 
Isoclinic lines connect those parts of the earth possessing the same 
magnetic inclination, and are represented on inclination charts. 'That one 
of these lines connecting those places where the needle is horizontal or has 
no inclination, is called the magnetic equator; to the north of this the north 
end of the needle will dip towards the earth, the reverse taking place to its 
south. In 1825 the one point of intersection of the magnetic and terrestrial 
equators was situated near the Island of St. Thomas, in the western coast 
of Africa, and distant 1883° from the node in the South Sea; from 1825 to 
1837 the former node has moved 4° towards the west. On the coast of 
Brazil the magnetic equator is 15° south of the terrestrial. A great 
advantage in investigating and establishing the laws of terrestrial magnetism 
is found in the fact, that all but about one fifth of the magnetic equator falls 
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