CLIMATE. 87

 

By carefully observing the decrease in pressure with the
elevation, at different altitudes, and making proper correc-
tions, the heights of mountains can be readily determined
by the barometer. The measurement of heights by the
barometer, or similar means, is called Hypsometry.

——00 $f,0-0—_—_.

CHAPTER. il,

Climate.

282. The Climate of a country is the condi-
tion of its atmosphere as regards heat or cold.

The climate of a country also embraces the con-
dition of the air as regards moisture or dryness,
and healthiness or unhealthiness, which are de-
pendent on the temperature. ;

233. Temperature——The temperature of the
atmosphere is determined by means of an instru-
ment called a thermometer.

The thermometer consists of a glass tube of very fine
bore, furnished at one end with a bulb. The tube is care-
fully dried and the bulb filled with pure mercury and
heated in the flame of a spirit-lamp; the mercury expands,
and, filling the fine capillary tube, a portion runs out
from the open end, thus effectually expelling the air. A
blowpipe flame is then directed against the open end and
the tube hermetically sealed. As the bulb cools, the mer-
cury contracts, and leaves a vacuum in the upper part of
the tube. The instrument will now indicate changes in
temperature; for, whenever the bulb grows warmer, the
column of mercury expands and rises; and when it grows
colder, it contracts and falls.

In order to compare these changes of level they are
referred to certain fixed or standard points: the freezing-
and boiling-points of pure water. These are obtained by
marking the respective heights to which the mercury rises
when the thermometer is plunged into melting ice and
into the steam escaping from boiling water. In Fuhren-
heit’s scale the freezing-point is placed at 32°, the boil-
ing-point at 212°, and the space between these two points
divided into 180, (212 —32) equal parts, called degrees. In
the Centigrade scale the freezing- and boiling-points are re-
spectively 0° and 100°. Fahrenheit’s degrees are repre-
sented by an F., thus, 212° F.; Centigrade’s by a C., as
100° C.

234. Astronomical and Physical Climates—
Astronomical climate is that which would result
were the earth’s surface entirely uniform and of
but one kind: all land or all water.

Physical climate is that which actually exists.

Since the physical climate is only a modification of the
astronomical, we shall briefly review the causes which
tend to produce a regular decrease in temperature from
the equator to the poles.

Astronomical Climate—The sun is practically
the only source of the earth’s heat. On account
of the earth’s spherical shape, those portions of

 

the surface are most powerfully heated. which re-
ceive the vertical rays, and these are confined to
a zone reaching 23° 27’ on each side of the equa-
tor. Beyond these the rays fall with an obliquity
which increases as we approach the poles.

235. Causes of the greater heating power of
the vertical rays of the sun than of the oblique
rays.

Je Id e ke Ja ! te ik

      

WE \
80. Causes of the Greater Heating Power of the Vertical
than of the Oblique Rays,

(1.) The vertical rays are spread over a smaller
area, Equal areas of the sun’s surface give off
equal quantities of heat. If, therefore, the bun-
dle of rays a 6, and ¢ d, come from equal areas,
the amounts of heat they emit will be equal; but
while the heat given off from a 0, the more ver-
tical rays, is spread over the earth’s surface from
J, to g, that from e d, is spread over the greater
area hi; the area f g, therefore, which receives
the more vertical rays, is much warmer than h j,
where the obliquity is greater.

(2.) The vertical rays pass through a thinner
layer of air. Only a part of the sun’s heat
reaches the surface of the earth; about 28 per
cent. of the vertical rays are absorbed during their
passage through the atmosphere. The amount of
this absorption must increase as the length of
path increases. In the figure, the light shading
represents the atmosphere. It is clear that the
oblique rays pass through a thicker stratum of
air than the more direct ones, and, therefore, are
deprived of a greater amount of heat.

According to Laplace, the thickness of the stratum of air
traversed by the rays when the sun is at the horizon is
35.5 times greater than when it is directly overhead. A

similar absorption of light affects the comparative bright-
ness of daylight in different latitudes.

(3.) The vertical rays strike more directly,
and, therefore, produce more heat. The heating