In Hale's quote from Act I, we see an example of Miller's clever word play. When Hale comes to town (after being sent for by Reverend Parris) he comes with large books filled with knowledge of the supernatural world.
Here are all your familiar spirits- your incubi and succubi; your witches that go by land, by air, and by sea; your wizards of the night and of the day.
Hale's books are "weighty" in two ways. 1) They are heavy. The large, over-sized books document all aspects of the witchcraft world. 2) The information in the books will condemn those who are witches and set free those who are not. At this point in the play, Hale trusts the books' knowledge and believes that armed with the information contained in them, he can find out if the girls are afflicted and find out who is tormenting them.