How does Henry David Thoreau use imagery in Civil Disobedience, and Other Essays?

Imagery:

"The slave-ship is on her way, crowded with its dying victims; new cargoes are being added in mid ocean; a small crew of slaveholders,countenanced by a large body of passengers, is smothering four millions under the hatches, and yet the politician asserts that the only proper way by which deliverance is to be obtained, is by 'the quiet diffusion of the sentiments of humanity,' without and 'outbreak.'" A Plea for Captain John Brown, p. 39