"I meant, of course," Hugh corrected himself, "when it does change from Wednesday to Thursday, it does it at midnight—and only at midnight."
"Surely," said Balbus. Lambert was silent.
"Well, now, suppose it's midnight here in Chelsea. Then it's Wednesday west of Chelsea (say in Ireland or America) where midnight hasn't arrived yet: and it's Thursday east of Chelsea (say in Germany or Russia) where midnight has just passed by?"
"Surely," Balbus said again. Even Lambert nodded this time.
"But it isn't midnight anywhere else; so it can't be changing from one day to another anywhere else. And yet, if Ireland and America and so on call it Wednesday, and Germany and Russia and so on call it Thursday, there must be some place—not Chelsea—that has different days on the two sides of it. And the worst of it is, the people there get their days in the wrong order: they've got Wednesday east of them, and Thursday west—just as if their day had changed from Thursday to Wednesday!"
"I've heard that puzzle before!" cried Lambert. "And I'll tell you the explanation. When a ship
