Some of these conjectures—about the shifting of the earth’s magnetic and geographic poles, about the fluid nature of the earth’s interior and its relation to surface structures—were remarkably prescient, identifying a research agenda that would keep geophysicists busy into the twenty-first century. During Franklin’s day the conjectures stimulated discussion among the members of the American Philosophical Society, where this letter was read and which met in Franklin’s library when he could not get out. And they showed his mind to be as active at eighty-two as it had been at forty-two.
