Ishmael thinks it’s surprising that, even though whales have been spouting and men have been watching them spout for 6,000 years, nobody really knows whether they spout water or air. He proposes to look into the matter.
Ishmael explains a little basic cetacean biology: whales have lungs like human beings instead of gills like fish, and they need to surface to breathe—but instead of breathing through their mouths, each whale breathes through a spiracle on top of its head.
Instead of breathing in and out constantly, the whale can get enough oxygen at once to last for a long time—and it can live without breathing until it needs to surface again.
Ishmael thinks this is partly the result of the labyrinthine system of veins and capillaries in the whale, which carry a supply of oxygenated blood the way a camel’s hump stores water.
As a result of its biology, the whale takes the same number of "breaths" (visible to the seamen as jets) each time it surfaces, and then it stays underwater ("sounds") for a set amount of time.
If hunters interrupt the whale and force it to "sound" before it’s taken enough breaths to completely oxygenate its blood, it won’t be able to stay under very long because it has to keep surfacing for air.
The whale’s respiratory system therefore makes it vulnerable to hunters.
Ishmael thinks about the limitations that this respiratory system has for whales: they have (as far as he can tell) no sense of smell and no voice. (Melville and other nineteenth-century whalemen didn’t know about whale song.)
But Ishmael still wants to know whether the whale’s breathing tube is just for air or also for water—perhaps it needs to discharge the water it takes in by mouth while feeding?
He knows it seems ridiculous to say he’s seen whales and can’t tell, but when there’s water splashing everywhere during a hunt, he really can’t, and even when the whale is calm, there is probably some water on top of the whale’s head.
Ishmael says that whalemen believe it’s dangerous to get too close to a whale’s spout—they think it blows out caustic acid.
Ishmael’s own hypothesis is that the spout blows out mist—a hilarious answer, because mist is by definition both air and water.
The mist that blows out of the whale’s head seems to Ishmael like the steam coming out of the ears of a philosophical genius.