Ishmael compares the heads of the sperm and right whales. He sees the sperm whale's forehead as an extremely efficient battering ram while the massive tun of oil in the sperm whale's head reminds him of the Heidelberg Castle wine cask, which has a capacity of 49,000 gallons. Tashtego falls into the whale's head and is rescued by Queequeg. The sperm whale's brain is relatively small, only a "nut."
Ishmael returns to one of his favorite topics, cetology, as he compares the two heads that are attached to the sides of the ship. Throughout the book, the narrator seems to thrive on contrasts: good vs. evil, white vs. black, starboard vs. port, God vs. Satan, madness vs. reason. However, it must be said that Ishmael seldom actually sees the world in such simplistic terms. Ultimately, opposites are only philosophical points of departure for the narrator. Neither Ahab nor Moby Dick, for example, can be limited to a single definition. The characters and issues here are complex, and Ishmael seems to delight in that rich view of life.
In these chapters, Ishmael's starting place is the contrast between the heads of the sperm and right whales. These are the only types of whale hunted by man, he tells us. Of the two, Ishmael feels that the sperm has more character; it is also much more valuable commercially. Its forehead is the world's most efficient natural battering ram, foreshadowing events at the end of the novel. The whale's eyes are small, reminding Ishmael of a colt's but lashless; they are placed on either side of the head so that the leviathan must get two distinct pictures of the world simultaneously, an ocular dichotomy that fits Ishmael's own initial view of life. He wonders how the whale blends the two outlooks. The ears are tiny, barely large enough to hold a writing quill. The sperm's ears have an outside opening; the right whale's ears are covered with a membrane.
Chapter 78 features an interesting literary device. Before long, it is apparent that Ishmael is more interested in the sperm whale. He enjoys the unique fact that the sperm's head can carry up to 500 gallons of valuable spermaceti oil. The image of Tashtego nearly drowning in it, rescued only by Queequeg, is an instance in which Melville uses dramatic action to illustrate a cetological point. It also breaks the monotony of what could be an excessively dry topic.
vacillate to sway to and fro, to waver in mind.
shoemaker's last a block or form shaped like a person's foot and used for making or repairing shoes.
for the nonce for the time being.
volition the act of using the will, a deliberate decision.
prairie a type of clam, here compared to the whale's head.
citadel a fortress, usually on a commanding height.