"Just how quickly does the earth's curve come into play, Father? I know the earth is round, of course, but, somehow, it seems so big that one never thinks of taking it into any practical account."
"It works mighty rapidly, my boy," said the old inspector. "You put a light right at sea level, on a day when there isn't a ripple on the sea, and five miles away, at sea level, you won't see a sign of it! Fifteen feet is the unit. Fifteen feet above sea level, you can see a light fifteen feet above sea level, seven miles away."
"Then why not build lighthouses like the Eiffel Tower, a thousand feet high!"
"Once in a while, Eric," his father said, rebukingly, "you talk absolutely without thinking. Didn't I just show you that the rays of a lantern had to be sent out in a single beam?"
"Yes, but what of that?"
"Can't you see that if your light is too high, the beam will have to strike the water at such an angle that its horizontal effect would be lost? That would mean that a ship could see the light seventy miles away, and lose it at fifty or forty miles from the lighthouse. No, boy, that wouldn't work. Tillamook Rock is quite high enough!"
"It does look high," agreed the boy, following his father's gaze to where, over the port bow, rose the menacing and forbidding reef on which the light stood.
"It's the meanest bit on the coast," said the inspector. "Wouldn't you say the sea was fairly smooth?"
"Like a mill-pond," declared Eric. "Why?"
"That just shows you," said his father. "You'd have to nail the water down to keep it from playing tricks around Tillamook. Look at it now!"