She was steaming near Ship Island, just below Memphis, when the boiler blew apart. Initial estimates at the time put the death toll at 250, and there was testimony at the board of inquiry that the Pennsylvania's engineer was not at his post at the time of the explosion, was instead "enjoying the company of women."
So do the calendar math: Sam leaves the Pennsylvania on June 5. The Pennsylvania explodes with great loss of life a week later, on the 13th.
Sam was lucky, but Henry not so much. He was horribly burned, and died a week later. And they said Sam Clemens never forgave himself for helping his brother get a job on that doomed riverboat.