“Cheerful!” said Pete. “What is this demonstrator? What’s it supposed to do?”

“Why, sir, it demonstrates the fourth dimension,” said Thomas. “It’s your uncle’s life work, sir.”

“Then let’s take a look at it,” said Pete. “Maybe we can support ourselves demonstrating the fourth dimension in shop windows for advertising purposes. But I don’t think Daisy will care for the career.”

Thomas marched solemnly to a curtain just behind the desk. Pete had thought it hid a cupboard. He slid the cover back and displayed a huge contrivance which seemed to have the solitary virtue of completion. Pete could see a monstrous brass horseshoe all of seven feet high. It was apparently hollow and full of cryptic cogs and wheels. Beneath it there was a circular plate of inch-thick glass which seemed to be designed to revolve. Below that, in turn, there was a massive base to which ran certain copper tubes from a refrigerating unit out of an ice box.

Thomas turned on a switch and the unit began to purr. Pete watched.

“Your uncle talked to himself quite a bit about this, sir,” said Thomas. “I gathered that it’s quite a scientific triumph, sir. You see, sir, the fourth dimension is time.”

“I’m glad to hear it explained so simply,” said Pete.

“Yes, sir. As I understand it, sir, if one were motoring and saw a pretty girl about to step on a banana peel, sir, and if one wished to tip her off, so to speak, but didn’t quite realize for—say, two minutes, until one had gone on half a mile—”

“The pretty girl would have stepped on the banana peel and nature would have taken its course,” said Pete.

“Except for this demonstrator, sir. You see, to tip off the young lady one would have to retrace the half mile and the time too, sir, or one would be too late. That is, one would have to go back not only the half mile but the two minutes. And so your uncle, sir, built this demonstrator—”