It would be easy and it would be amusing to go on with examples of mixed figures and figures which are ineffective, but the point hardly needs further illustration.
Pushing a figure too far is a fault less common in these days than it has been at some periods of our literary history when fashions in writing were more ornate than at present. If a writer realizes what a simile means, he is not likely to fall into this error. It is when he introduces a figure for the sake of the figure, and not for the purpose of strengthening or making more clear what he is saying, that this fault occurs.
These lines of Cowper may serve as an example:
Man is a harp, whose chords elude the sight,
Each yielding harmony disposed aright;
The screws reversed (a task which, if He please,
God in a moment executes with ease),
Ten thousand thousand strings at once go loose,
Lost, till He tune them, all their power and use.
If this stopped with the second line, it might do well enough; but when the attention is forced to the consideration of the mechanical details of the harp, and the image of ten thousand thousand strings and a corresponding number of screws, and the notion applied to a man bereft of his wits, the idea becomes absurd, and whatever value the figure might have is entirely lost.
A clear realization of what he is doing will also prevent the writer from mingling figure and fact. “He was the guardian genius of Ireland, and had served with eloquence and credit in legislative halls,” could hardly have been written by one who felt clearly the meaning and significance of the figure. To realize how a guardian genius would look in legislative halls would have brought him at once to his senses. It is always necessary to have sharply defined in the brain whatever one is saying, but this is especially true of any use of language which invites the reader to loose his grasp upon absolute, literal fact.
The difference between simile and metaphor is one which need not be pressed very sharply. It is to be observed that as writing becomes more excited or impassioned there is less need of insisting upon formalities; so that as the writer warms his readers, he may assume a likeness instead of explicitly stating it. At the beginning of a passage it may be better to say, “Napoleon swept like a tempest over Europe,” whereas later, the reader having become interested in the theme, it is fitting to write, “Napoleon, the tempest which was sweeping over Europe.” There is probably no better rule than for the writer to do that which at the moment seems to him most natural, and then in revision to see if it strikes him as it did when he wrote it.
Personification may be conveniently regarded as classed with simile and metaphor. It is somewhat out of fashion, but if it is used it is to be governed by the rules given above. One who realizes what he is saying in the phrase, “Hope told a flattering tale,”—who sees that he is representing Hope as a beautiful and seductive being,—is not likely to go on to add, “but this hope was founded upon a delusion,” because he cannot conceive a young nymph or goddess as being founded upon anything. He will naturally and without effort carry out the figure, and say, “but she beguiled us;” or, “but all her flatteries were delusions.” The truth is, that the mind will generally go in the right direction if it is given a fair chance. It is only when we hamper it with rules not understood, when we force it to go in paths which we suppose to be laid out by conventions, or when we endeavor to make it pace according to our vanity, that it goes astray. Be natural in the use of figures, and you will seldom be wrong.