How is that to be done?  The answer is difficult, and like that concerning the use of figurative language:  do not try for it too deliberately.  If without your thinking of it you find yourself becoming more earnest in speech, and more impressed with the seriousness of the issue you are arguing, your voice will show it naturally.  So when you are writing:  your earnestness will show, if you have had the training and have the natural gift for expression in words, in a lengthening and more strongly marked rhythm, in an intangibly richer coloring of sound.  In speech the rhythm is apt to be shown in what is called parallel structure, the repetition of the same form of sentence, and in rhetorical questions.  In writing, these forms more easily tend to seem either excited or artificial.  Sustained periodic structure, too, can be carried by the speaking voice, when it would lag if written.  Every one recognizes this incommunicable thrill of eloquence in great speakers and writers, but it is so much a gift of nature that it is not wise consciously to cultivate it.

59.  Fairness and Sincerity.  In the long run, however, nothing makes an argument appeal more to readers than an air of fairness and sincerity.  If it is evident in an argument of fact that you are seeking to establish the truth, or in an argument of policy that your single aim is the greatest good of all concerned, your audience will listen to you with favorable ears.  If on the other hand you seem to be chiefly concerned with the vanity of a personal victory, or to be thinking of selfish advantages, they will listen to you coolly and with jealous scrutiny of your points.

Accordingly, in making your preliminary survey to prepare the statement of the facts that are agreed on by both sides, go as far as you can in yielding points.  If the question is worth arguing at all you will still have your hands full to get through it within your space.  In particular waive all trivial points:  nothing is more wearisome to readers than to plow through detailed arguments over points that no one cares about in the end.  And meet the other side at least halfway in agreeing on the facts that do not need to be argued out.  You will prejudice your audience if you make concessions in a grudging spirit.  Likewise, wherever you have, to meet arguments put forward by the other side, state them with scrupulous fairness; if your audience has any reason to suppose that you are twisting the assertions of the other side to your own advantage, you have shaken their confidence in you, and thereby weakened the persuasive force of your argument.  Use sarcasm with caution, and beware of any seeming of triumph.  Sarcasm easily becomes cheap, and an air of triumph may look like petty smartness.

In short, in writing your argument, assume throughout the attitude of one who is seeking earnestly to bring the disagreement between the two sides to an end.  If you are dealing with a question of fact, your sole duty is to establish the truth.  If you are dealing with a question of policy, you know when you begin that whichever way the decision goes, one side will suffer some disadvantage; but aim to lessen that disadvantage, and to discover a way that will bring the greatest gain to the greatest number.  An obvious spirit of conciliation is a large asset in persuasion.