"Sometimes," Balbus gently insisted. "Six midnights out of seven, it changes to some other name."
"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 goes round the world from east to west, we know that it loses a day in its reckoning: so that when it gets home, and calls its day Wednesday, it finds people here calling it Thursday, because we've had one more midnight than the ship has had. And when you go the other way round you gain a day."
"I know all that," said Hugh, in reply to this not very lucid explanation: "but it doesn't help me, because the ship hasn't proper days. One way round, you get more than twenty-four hours to the day, and the other way you get less: so of course the names get wrong: but people that live on in one place always get twenty-four hours to the day."
"I suppose there is such a place," Balbus said, meditatively, "though I never heard of it. And the people must find it very queer, as Hugh says, to have the old day east of them, and the new one west: because, when midnight comes round to them, with the new day in front of it and the old one behind it, one doesn't see exactly what happens. I must think it over."
So they had entered the house in the state I have described—Balbus puzzled, and Lambert buried in gloomy thought.