"Say fifteen, sir," said Dubberley magnanimously.
"Mr Dubberley," said Robin, in a voice which made us all jump, "have you a bathroom in your house?"
Even the Twins, who were growing a trifle lethargic under the technicalities of the subject, roused themselves at this fresh turn of the conversation.
"Ah—eh—I beg your pardon?" said Dubberley, a trifle disconcerted.
"A bathroom," repeated Robin—"with a good long bath in it?"
"Er—yes."
"Well, Mr Dubberley, when you go home this afternoon, go upstairs and fill the bath with water. Then take a large bath-sponge—something nearly as broad as the bath—and sweep it along from end to end, about a foot below the surface, at a speed of fifteen miles per hour—that will be twenty-two feet per second—and see what happens!"
Robin had unconsciously dropped into what I may call his debating-society manner. His chin stuck out, and his large chest heaved, and he scooped the air, as if it had been the water of Dubberley's bath, with one of Kitty's priceless Worcester teacups.
Dubberley sat completely demoralised, palpitating like a stranded frog.
"Then," continued Robin, "you would observe the result of placing a partly submerged and rapidly moving body in a shallow and restricted waterway. You would kick half the water right out of the canal to begin with, and the other half would pile itself up into a wave under your bow big enough to offer an almost immovable resistance to the progress of your vessel."