Sound waves can travel only in the air

In the fourth paragraph, the author mentions what he "gained from the dialogue” and what he "got from Sheridan” (lines 29–30) primarily to <u>illustrate his developing arguments against slavery and validate his beliefs</u>

Although your question is incomplete, I made use of the context clues to come to a conclusion of the most plausible meaning based on the words shown and this was to validate his beliefs about the issue of slavery.

<h3>What is a Narration?</h3>

This refers to the storytelling that is done with the aid of a narrator and this helps to advance the plot.

Hence, we can see that In the fourth paragraph, the author mentions what he "gained from the dialogue” and what he "got from Sheridan” (lines 29–30) primarily to <u>illustrate his developing arguments against slavery and validate his beliefs</u>

Although your question is incomplete, I made use of the context clues to come to a conclusion of the most plausible meaning based on the words shown and this was to validate his beliefs about the issue of slavery.