Ishmael attempts to use physiognomy, the nineteenth-century pseudoscience of reading facial features to determine character (think of it as palm-reading for your face), to learn something about the head of the sperm whale.
The most important thing about the whale’s head, Ishmael decides, is that it doesn’t have a nose – but this ends up being good, because a nose on a whale would be "impertinent" (19.2).
The whale’s brow is royal and majestic, its forehead broad and high, which gives it a royal dignity. (A high forehead was considered a sign of intelligence by many physiognomists.)
Ishmael knows the reader will object and say that the sperm whale can’t be a "genius," but he thinks the whale’s silence is its greatest genius.
After all this, though, Ishmael decides that it’s impossible to really use a face and skull to decipher a creature’s nature.
He dismisses physiognomy as "a passing fable" and, presenting the whale’s brow to the reader, says "Read it if you can" (79.6).