In all good writing we find a similar dependence in thought.  Each sentence takes a meaning because of its relation to some other.  The personal pronouns and pronominal adjectives, adverbial phrases indicating time or place, conjunctions, and such expressions as certainly, however, on the other hand, etc., are used to indicate more or less directly a relation in thought between the phrase or sentence in which they occur and some preceding one.  If the reader cannot readily determine to what they refer, the meaning becomes obscure or ambiguous.  The pronominal adjectives and the personal pronouns are especially likely to be used in such a way as to cause ambiguity.  Care must be taken to use them so as to keep the meaning clear, and your own good sense will help you in this more than rules.  Notice in your reading how frequently expressions similar to those mentioned above are used.

+Theme XVII.+—­Write a theme suggested by one of the following subjects:—­

1.  The last quarter. 2.  An excursion with the physical geography class. 3.  What I saw while riding to town. 4.  The broken bicycle. 5.  An hour in the study hall. 6.  Seen from my study window.

(Are your sentences so arranged that the relation in thought is clear?  Are the personal pronouns and pronominal adjectives used so as to avoid ambiguity?  Does your story relate real events or imaginary ones?  If imaginary events are related, have you made them seem probable?)

+37.  Getting the Main Thought.+—­In many cases the relation in thought is not directly indicated, and we are left to determine it from the context, just as we decide upon the meaning of a word because of what precedes or follows it.  In this case the meaning of a particular sentence may be made clear if we have in mind the main topic under discussion.  Many pupils fail in recitations because they do not distinguish that which is more important from that which is less so.  If a dozen pages of history are assigned, they cannot master the lesson because it is too long to be memorized, and they are not able to select the three or four things of importance with which it is really concerned.  Thirty or forty minor details are jumbled together without any clear knowledge of the relations that they bear either to one another or to the main thoughts of the lesson.

In the following selection but three things are discussed.  Determine what they are, but not what is said about them.

In all the ages the extent and value of flood plains have been increased by artificial means.  Dikes or levees are built to regulate the spread and flow of the water and to protect the land from destructive floods.  Dams and reservoirs are constructed for the storage of water, which is led by a system of canals and ditches to irrigate large tracts of land which would be otherwise worthless.  By means of irrigation, the farmer has control of his water supply and is able to get larger returns than are possible where he depends upon the irregular and uncertain rainfall.  It is estimated that in the arid regions of western United States there are 150,000 square miles of land which may be made available for agriculture by irrigation.  Perhaps in the future the valley of the lower Colorado may become as productive as that of the Nile.