SYLLABUS. 115

 

strata of air, which gradually increase or decrease
in density. In this way the objects appear either
inverted or erect, but always out of their true
position. Sometimes the objects are repeated, one
being seen above the other. The mirage occurs
both over water and land. It is caused by the
turning of the rays of light out of their original
direction.

The Mirage of the Desert occurs over hot, arid
surfaces, whenever the strata of air increase rap-
idly in density from the surface upward. The
rays of light from distant objects, such as trees,
are reflected from one of the lower layers of air,

and, entering the eye of the observer, appear to
come from inverted objects, which seem to be
surrounded by a sheet of water. The image of
a real tree is seen, but out of its true situation, so
that when the observer reaches the place he finds
nothing. |

The mirage frequently occurs on the sea. Ves-
sels that are too far below the horizon to be di-
rectly visible, become visible by refraction. This
phenomenon is called looming. The vessels are
seen both erect and inverted, and sometimes ap-
pear suspended in the clouds. Distant islands
are sometimes visible from the same cause.

RR III

SYLLABUS.

——050400——_

The rapidity of evaporation increases—1. With the tem-
perature of the atmosphere. 2. With the extent of sur-
face exposed. 3. With a decrease in the quantity of va-
por already in the air. 4. With the renewal of the air;
and, 5. With a decrease of the pressure on the surface.

When air can hold no more moisture in an invisible
state, it is said to be saturated or at its dew point.

Whenever the air is lowered below the temperature of
its dew point, its moisture is deposited as cloud, mist,
snow, hail, sleet, or rain.

More dew is deposited on clear nights, when the wind is
moderate, than on cloudy nights, when the wind is high.

In fogs, mists, and clouds, the moisture is condensed as
minute drops.

Clouds owe their variety of forms to the action of aérial
currents, and their constant tendency to settle.

The dense fogs so common off the banks of Newfound-
land are caused by the chilling of the warm, moist air of
the Gulf Stream by the cooler air of the Labrador cur-
rent.

The primary forms of clouds, are the cirrus, the cumu-
lus, the nimbus, and the stratus.

The secondary forms of clouds, are the cirro-stratus, the
eirro-cumulus, and the cumulo-stratus.

Rain falls whenever the temperature of a mass of air is
lowered considerably below the temperature of its dew
point.

This reduction of temperature may occur—1l. By a
change of altitude by means of ascending currents. 2.

' By a change of latitude, as by the warm equatorial cur-
rents flowing into colder regions nearer the poles. 3. A
comparatively small rainfall. may be caused by the inter-
mingling of moist cold and moist warm air.

As a rule, the equatorial currents bring rain, the polar
currents, drought.

In the zone of calms, it rains during the hottest part of
the day, or in the afternoon, when the ascending currents
are strongest. :

In the zone of the trades, it rains during the hottest
part of the year, or in summer.

14

Between lat. 24° and 30°, both N. and S., the rainfall is
scanty, and in some localities almost absent.

In the zone of the variables, it may rain at any hour of
the day or night, or at any time of the year.

In the polar zones, the winters are clear; snows and
drizzling rains occur in spring and autumn.

Between lat. 30° and 35°, both N. and S., it is dry in
summer during the prevalence of the polar currents. The
rest of the year is wet.

The rainfall of any place is determined by means of an
instrument called a rain-gauge or pluviometer.

An inch of rain on the surface of a square yard is equal

in weight to 46.75 pounds; an inch on the surface of an

acre, to the weight of about 100 tons.

The quantity of rain decreases from the equator to the
poles, and from the coasts of the continents toward the
interior.

More rain falls on mountains than on plains; more on
plains than on plateaus; more in the Northern Hemi-
sphere than in the Southern.

In the tropics of the New World, the annual rainfall is
115 inches; in the Old World, only 77 inches.

In the temperate regions of the New World, the annual
rainfall is 35 inches; in the Old World, but 34 inches.

The average rainfall of Europe, betveen lat. 36° and 60°
N., is 34 inches.

The average rainfall in the United States, between 24°
30’ and 45° north latitude, is 39 inches.

The desert belt of the eastern continent extends from
the western shores of Northern Africa eastward to the
Great Kinghan Mountains in Asia. It includes the Sa-
hara, the Arabian and Persian Deserts, and the Desert of
Mongolia. The aridity of this immense tract is caused by
the absence of rain. ;

The desert tracts near the summits of high mountains
are caused by the absence of heat and liquid moisture.

Hail falls when bodies of warm and intensely cold air
are rapidly commingled.

Snow falls when the moisture is condensed at tempera-
tures at or helow 32° Fahr., under conditions favorable to