Then I understood.

Through the cracks in the bottom of the upper bunk he had set fire to the mattress.

He still retained sufficient use of his left arm to do this.

The damp straw of the mattress, fired from beneath and denied air, had been smouldering all the while.
As I dragged the mattress out of the bunk it seemed to disintegrate in mid-air, at the same time bursting into flames.

I beat out the burning remnants of straw in the bunk, then made a dash for the deck for fresh air.
Several buckets of water sufficed to put out the burning mattress in the middle of the steerage floor; and ten minutes later, when the smoke had fairly cleared, I allowed Maud to come below.