2015考研英语作文素材精选 (十一)
考研英语作文是一个考察综合运用语言的部分，需要同学们运用逻辑思维下笔成文，因此，考前看一些意义深远、质量好的文章很有必要。以下是2015考研英语作文备考素材精选，请作参考。2015考研英语作文素材精选 (十一)
39 The Salinity of Ocean Waters
If the salinity of ocean waters is analyzed, it is found to vary only
slightly from place to place. Nevertheless, some of these small changes are
important. There are three basic processes that cause a change in oceanic
salinity. One of these is the subtraction of water from the ocean by means of
evaporation--- conversion of liquid water to water vapor. In this manner the
salinity is increased, since the salts stay behind. If this is carried to the
extreme, of course, white crystals of salt would be left behind.
The opposite of evaporation is precipitation, such as rain, by which water
is added to the ocean. Here the ocean is being diluted so that the salinity is
decreased. This may occur in areas of high rainfall or in coastal regions where
rivers flow into the ocean. Thus salinity may be increased by the subtraction of
water by evaporation, or decreased by the addition of fresh water by
precipitation or runoff.
Normally, in tropical regions where the sun is very strong, the ocean
salinity is somewhat higher than it is in other parts of the world where there
is not as much evaporation. Similarly, in coastal regions where rivers dilute
the sea, salinity is somewhat lower than in other oceanic areas.
A third process by which salinity may be altered is associated with the
formation and melting of sea ice. When sea water is frozen, the dissolved
materials are left behind. In this manner, sea water directly materials are left
behind. In this manner, sea water directly beneath freshly formed sea ice has a
higher salinity than it did before the ice appeared. Of course, when this ice
melts, it will tend to decrease the salinity of the surrounding water.
In the Weddell Sea Antarctica, the densest water in the oceans is formed as
a result of this freezing process, which increases the salinity of cold water.
This heavy water sinks and is found in the deeper portions of the oceans of the
world.
NOTE：
salinity / sэ'linэti; sэ`linэti/
n the high salinity of sea water 海水的高含盐量.
--->>>saline / 'seilain; US -li:n; `selin/
1.adj (fml 文) containing salt; salty 含盐的; 咸的:
* a saline lake 盐湖 * saline springs 盐泉
* saline solution, eg as used for gargling, storing contact lenses, etc
盐溶液(如用于漱喉、存放隐形眼镜等).
2. n (medical 医) solution of salt and water 盐水.
40 Cohesion-tension Theory
Atmospheric pressure can support a column of water up to 10 meters high.
But plants can move water much higher; the sequoia tree can pump water to its
very top more than 100 meters above the ground. Until the end of the nineteenth
century, the movement of water in trees and other tall plants was a mystery.
Some botanists hypothesized that the living cells of plants acted as pumps. But
many experiments demonstrated that the stems of plants in which all the cells
are killed can still move water to appreciable heights. Other explanations for
the movement of water in plants have been based on root pressure, a push on the
water from the roots at the bottom of the plant. But root pressure is not nearly
great enough to push water to the tops of tall trees. Furthermore, the conifers,
which are among the tallest trees, have unusually low root pressures.
If water is not pumped to the top of a tall tree, and if it is not pushed
to the top of a tall tree, then we may ask: how does it get there? According to
the currently accepted cohesion-tension theory, water is pulled there. The pull
on a rising column of water in a plant results from the evaporation of water at
the top of the plant. As water is lost from the surface of the leaves, a
negative pressure, or tension, is created. The evaporated water is replaced by
water moving from inside the plant in unbroken columns that extend from the top
of a plant to its roots. The same forces that create surface tension in any
sample of water are responsible for the maintenance of these unbroken columns of
water. When water is confined in tubes of very small bore, the forces of
cohesion (the attraction between water molecules) are so great that the strength
of a column of water compares with the strength of a steel wire of the same
diameter. This cohesive strength permits columns of water to be pulled to great
heights without being broken.
页:
[1]