The phrase that gives the sentence a humorous laughing voice is;
In the passage, "A Witch Trial at Mount Holly," Benjamin Franklin describes a practice in his time when men and women will be accused of being witches.
They are subjected to several tests and the test in this passage was them being weighed against a Bible. It was believed that the weight of the Bible will supersede those of the witches and so, they will be thrown off.
Franklin describes these events in a comical manner to describe the folly of what they were doing.
When he said that they searched themselves to see if something of weight will be found among them, especially pins he was mocking them because pins are not known to be weighty.
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