1968 Like a knife blade, the year severed past from future

In the early months of the year, rivers overflowed their banks. "A spirit of change and recklessness seemed to pervade the very inhabitants of the forest," a naturalist wrote afterward. Squirrels inexplicably marched southward in migration, tens of thousands at a time. They plunged heedlessly into the Ohio River and drowned. Earthquakes reversed the flow of the Mississippi so that its waters surged upstream at the speed of galloping horses. Whole forests fell down, like stacked fields of rifles toppling. A double-tailed comet appeared in the night sky over America.

All of that happened in the annus mirabilis of 1811, a...