g shutter
with wide slits in it?"

"That was the old idea. We use it still as a cheap way of changing a
fixed light to one with a definite character. It works all right, only
it's a waste of power to have the light darkened part of the time. Then,
too, if the shutter revolves too quickly, the light is like little
flashes of lightning, while, if it goes too slowly, a lookout might
happen to scan that point on the horizon at the instant it was dark. In
that way the value of the warning would be lessened."

"I know the flashing light is quite different, Father, but just how is
it worked?" asked the boy. "It's because of some arrangement of the
lens, isn't it?"

"Exactly. Light travels in straight lines in every direction. One of the
problems of illumination in lighthouse work is to make all these beams
come to one focus. We don't want to light the sky, nor the sea at the
foot of the lighthouse. So a first-order light is built up of rows on
rows of prisms so arranged that the light will be refracted from every
direction to one point. An ordinary student's reading lamp, inside a big
lighthouse lens, would give a light that could be seen a good many
miles!"

[Illustration: REFILLING PINTSCH GAS BUOY.

Courtesy of Safety Car Lighting Co.]

[Illustration: LIGHTHOUSE TENDER APPROACHING BUOY.

Courtesy of Safety Car Lighting Co.]

"That is, if it were high enough up."

"Of course."

"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 i