Notice the following example:—­

The effect of the polar whirls may be seen in the rapid rotation of water in a pan or bowl.  The centrifugal force throws the water away from the center, where the surface becomes depressed, and piles it up around the sides, where the surface becomes elevated.  The water being deeper at the sides than at the center, its pressure upon the bottom is proportionately greater.  A similar effect is produced by the whirl of the air around the polar regions.  It is thrown away from the polar regions and piled up around the circumference of the whirl.  There is less air above the polar regions than above latitude 30 deg.-40 deg., and the atmospheric pressure is correspondingly low at one place and high at the other.  Thus the centrifugal force of the polar whirl makes the pressure low in spite of the low temperature.  The position of the tropical belts of high pressure is a resultant of the high temperature of the equatorial regions on one side and the polar whirls on the other.

—­Dryer:  Lessons in Physical Geography.

+Theme XCV.+—­Write an expository theme using cause or effect.

Suggested subjects:—­

1.  The causes of the French Revolution. 2.  How ravines are formed. 3.  Irrigation. 4.  Effects of smoking. 5.  Lack of exercise. 6.  Volcanic eruptions.

(Did you find it necessary to make use of any other method of explanation?  Did you make use of description in any place?)

SUMMARY

1.  Exposition is that form of discourse the purpose of which is to
    explain.

2.  The essential characteristics of an exposition are—­
    a. That it possess unity because it contains only those facts
          essential to its purpose.
    b. That the facts used be arranged in a coherent order.

3.  Exposition is concerned with (a) general terms or (b) general
    propositions.

4.  The steps in the exposition of a term are—­
    a. Definition.  This may be—­
          (1) By synonym (inexact).
          (2) By use of the logical definition (exact).
    b. Division.  This may be—­
          (1) Complete (classification).
          (2) Incomplete (partition). 
         The same principle of division should be followed throughout.

5.  Exposition of a proposition may use any one of the
following methods—­
    a. By repetition.
    b. By giving examples.
    c. By stating comparisons and contrasts.
    d. By making obverse statements.
    e. By relating particulars or details.
    f. By stating cause or effect.
    g. By any suitable combination of these methods.

XI.  ARGUMENT

+170.  Difference between Argument and Exposition.+—­Argument differs from exposition in its purpose.  By exposition we endeavor to make clear the meaning of a proposition; by argument we attempt to prove its truth.  If a person does not understand what we mean, we explain; if, after he does understand, he does not believe, we argue.