110 PHYSICAL GEOGRAPHY.

 

In the Arctic zone glaciers are particularly
numerous and extensive. Here they generally
reach down into the sea. They are found in the
islands of the Arctic Archipelago, in Greenland,
Iceland, Jan Mayen, and Spitzbergen.

The Humboldt Glacier, in Greenland, is sixty-nine miles
broad at its lower extremity in the sea. In all the Arctic
glaciers, the névé region is more extended than in those
of more southern latitudes. The terminal moraines are
found at the bottom of the sea, near the foot of the
glacier. fe

In the lofty mountain-ranges of the Himalayas and in
the Karakorum, occur other less known, though exten-
sive, regions of glaciers.

288. Icebergs— When the glacier extends into
the sea, the base is undermined by the warmer
waters of the ocean, and great fragments are
broken off by the waves, forming floating moun-
tains of ice, called icebergs. Icebergs are narticu-
larly numerous in the North Atlantic, iy which
they descend from the extensive Arctic glacial
region already described.

The limits of the Arctic and Antarctic drift ice are
shown in the map of the isotherms.

Fig, 98, Icebergs,
The ice floes of the polar seas have their origin
in the snow which falls into the cold water, re-
maining partially dissolved and subsequently

freezing, thus adding to the thickness of the ice
formed. ;

 

289. The Glacial Epoch of the Earth.—Toward the
close of the Mammalian Age, a change occurred in the cli-
mate of the earth, and extensive glaciers covered most of
the northern continents, reaching, in many instances, far
toward the south. In the United States, their southern
limit appears to have been at about lat. 39° N., in Southern
Pennsylvania, Ohio, Indiana, Illinois, and Iowa. In Eu-
rope, they extended as far south as the 50° N. lat. In
South America, they probably extended as far toward the
equator as 41° S. lat.

The evidences of the existence of ancient glaciers are
found in the presence of accumulations of unstratified
material, called the drift; in the presence of old moraines; ©
in glacial scratches and grooves on rocky slopes; in eroded
valleys; and in the presence of numerous large boulders,
which are found at great distances from their places of
origin.

—_-0594 00o—_——.

CHAPTER Jd;

Electrical and Optical Phenomena.

290. Nature of Electricity —Electricity is now
generally believed to be, due to a peculiar wave
motion in the luminiferous ether, the medium
which transmits the waves of light and heat.

When a body is electrified it acquires a certain
power of doing work, called electric potential.
Electric potential is measured in units called
volts. The path through which an electric dis-
charge passes is called the circuit. All circuits
offer a measurable resistance to the passage of an
electric discharge. Electric resistance is meas-
ured in units called ohms.

The rate at which electricity passes through a
circuit is called the current, and is measured in
units called ampéres. An ampére is the current
which would pass in a circuit whose resistance is
one ohm, under a potential of one volt.

Though electricity is probably not a fluid, yet it resem-
bles a fluid in many respects, and the units already re-
ferred to are, to a certain extent, based on this resem-
blance. The quantity of liquid that flows through a pipe
in a given time depends on the pressure on the liquid, and
the resistance offered by the pipe. The quantity-per-sec-
ond corresponds to the ampéres ; the pressure which causes

the flow, to the volts; and the resistance which limits the
flow, to the ohms.

Electricity may be produced in bodies by a
variety of causes: such as friction, heat, chemical
action, magnetism, and animal or vegetable life.

There are two distinct forms of electrical excitement:
the positive and the negative. A body with a high potential
is generally assumed to be positively charged; one with a
low potential, negatively charged. The current is assumed
to flow from the higher to the lower potential, or from the
positive to the negative. Bodies charged with electricity