The Boy With the U. S. Life-Savers

by Francis Rolt-Wheeler

"Why do the railroads use red for danger signals, then?"

"Habit, mainly. It's wrong, of course, and a good many of the railroads are changing their danger signals from red to yellow. So far as we're concerned in the Lighthouse Service, however, we're getting rid of all the fixed red lights wherever a long-range warning is needed."

"How do you distinguish the different lights, then?"

"Using flashing lights, with flashes of different duration."

"Why didn't you always do that?" asked Eric.

"Didn't know enough," was the simple reply. "It's only lately that we've found out how to work a flashing light without any loss of power. In the old days we used to depend on occulting lights, but now, flashing lights are much more powerful. You know the difference?"

"Sure! An occulting light means that some of the time the light is shut off, and at others it isn't. Wasn't it worked by a revolving 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!"