what is the humor in this chapter

The catching of the whale in these chapters has two humorous possibilities. The first is that the laws of England ask that the head be given to the kind and the tail to the queen - the idea is that there is little left for the actual whale hunter himself. The second bit of humor happens when Stubb convinces the captain that the "sick" whales might be dangerous to the crew. When he does convince the captain of this, the captain dumps one which Stubb recovers as soon as the ship is out of sight. Then Stubb actually captures the whale and finds the valuable ambergris inside the whale's dead body.