GENERAL

 

SYLLABUS. 165

 

are subjected by the rains, their surfaces are cleansed of
the decaying animal and vegetabie matters which cover
them.

The drainage of the land is of two kinds: subterranean
and surface drainage.

Surface drainage is either oceanic or inland.

According to the size of their reservoirs, springs are
either constant or temporary ; according to the depth of the
reservoirs, they are either cold or hot; according to the
nature of the mineral substances lining their reservoirs,
they become charged with various mineral substances; if
their reservoirs discharge through a siphon-shaped tube,
they are periodical; if their reservoirs are formed of con-
cave layers, they are called artesian springs.

The quantity of water discharged by a river depends—
1. On the size of its basin. 2. On the amount of its rain-
fall. 3. On the climate of its basin, a dry, hot air dimin-
ishing the quantity by evaporation. 4. On the nature of
jts bed or channel, whether leaky or not. 5. On the
features of its basin, whether wooded or open.

The material eroded by a river is deposited—i. In the
channel of the river. 2. On the alluvial flats or flood-
grounds, 3. At the mouth, 4. Along the coast near the
mouth.

In the upper courses of rivers erosion occurs mainly on
the bottom of the channel; in the lower courses, at the
sides.

The Atlantic and Arctic Oceans receive the waters of
nearly all the large river systems of the world.

Lakes connected with the system of oceanic drainage are
generally fresh; those connected with the inland drainage
are generally salt.

The bed of the ocean is less diversified than the surface
of the land.

The greatest depth of the ocean is probably greater than
the greatest elevation of the land.

The articulation of land and water assumes four dis-
tinct forms,—inland seas, border seas, gulfs and bays,
and fiords.

Inland seas characterize the Atlantic; border seas, the
Pacific; gulfs and bays, the Indian; fiords, the Atlantic
and Pacific.

A deposit of fine calcareous mud or ooze, formed of the
hard parts of minute animalcule, occurs over extended
areas of the floor of the ocean.

Tides are caused by the attraction of the sun and moon;
spring tides by their combined attractions; neap tides, by
their opposite attractions.

Constant ocean currents are occasioned by the heat of the
sun and the rotation of the earth.

The vertical rays of the sun are warmer than the oblique
rays—l. Because they have a less depth of air to pass
through. 2. Because they are spread over a smaller area.
3. Because, striking the surface more directly, they produce
greater heat. :

Continual summer characterizes the tropics ; summer and
winter of nearly equal duration, the temperate zones ; and
short, hot summers, followed by long, intensely cold win-
ters, the frigid zones.

The irregular distribution of heat over the earth is
caused—1. By the irregularities of the surface. 2. By pecu-
liarities in the distribution of the land- and water-areas.
3. By the influence of the winds and ocean currents. 4.
By the nature of the surface.

Winds are caused by the disturbance of the equilibrium
of the atmosphere by heat.

The general motion of the surface winds is towards an

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area of greatest heat; of the upper currents, towards an
area of least heat.

The general atmospheric circulation is from the equator
to the poles, and from the poles to the equator.

In storms, the wind has a rotary motion around an area
of low barometer, which, at the same time, progresses along
the surface.

In the northern hemisphere, the rotary motion is in an
opposite direction to the hands of a clock ; in the southern
hemisphere, in the same direction as the hands of a clock.

Moisture may be precipitated from the air in the form of
dew, mist, fog, cloud, rain, hail, sleet, or snow.

In order that any form of precipitation may occur, the
air must be reduced below the temperature of its dew-
point.

Glaciers are immense masses of ice and snow, which
move with extreme slowness down the higher valleys of
mountain-ranges. They resemble rivers in that they re-
ceive through the drainage of their basins, the solid
material which flows into them.

The snow line is the distance above the sea where the
snow remains throughout the year. The height of its
lower level above the sea depends (1.) On the amount of
the snowfall. (2.) On the temperature of the valley. (3.)
On the inclination of the slope.

The unit of electric potential is called a volt; the unit
of current an ampére; the unit of resistance an ohm.
Comparing the flow of electricity to a current of water in
a pipe, the volt corresponds to the pressure causing the
flow, the ohm to the resistance, or friction, opposing it,
and the ampére to the quantity of flow per second.

The principal electrical phenomena of the atmosphere are
thunder and lightning, St. Elmo’s fire, and the aurora.

The principal optical phenomena are the rainbow, the
mirage, halos, and corone.

The earth acts like a huge magnet. Its magnetism is
probably due to the circulation around it of electrical cur-
rents, generated by the sun’s heat.

The true basis for the distribution of vegetation is the
distribution of the light, heat, and moisture, upon which
its existence mainly depends.

The variety and luxuriance of vegetation decrease as
we pass from the equator to the poles, or from the base
of a mountain to the summit.

The principal food-plants of the tropical regions are rice,
bananas, plantains, dates, cocoa-nuts, cassava, bread-fruit,
sago, and yams.

Coffee, tea, cocoa, pepper, cloves, nutmegs, and vanilla
are also products of the tropics.

The principal food-plants of the temperate zones are
barley, rye, wheat, oats, maize or Indian corn, buckwheat,
and the potato.

Animals are restricted, by conditions of food and climate,
to certain regions of the earth. '

They are dependent for their continued existence upon
plants, the distribution of which therefore forms an excel-
lent basis for the distribution of animals.

With a few exceptions, animals possess but little power
of becoming acclimated, or living in a climate differing
greatly from that in which they were created.

The grassy meadows and prairies in North America cause
the fauna of the continent to be characterized by a pre-
ponderance of plant-cating mammals. Its extensive lake-
and river-systems harbor a great number and variety of
waterfowl.

South America is characterized by the predominance of
its reptiles and insects. Birds are also numerous.

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