Upon each of these wharf-boats the association's officers placed a strong box fastened with a peculiar lock which was used in no other service but one--the United States mail service.

It was the letter-bag lock, a sacred governmental thing.

By dint of much beseeching the government had been persuaded to allow the association to use this lock.

Every association man carried a key which would open these boxes.

That key, or rather a peculiar way of holding it in the hand when its owner was asked for river information by a stranger--for the success of the St.Louis and New Orleans association had now bred tolerably thriving branches in a dozen neighboring steamboat trades--was the association man's sign and diploma of membership; and if the stranger did not respond by producing a similar key and holding it in a certain manner duly prescribed, his question was politely ignored.