The trio were climbing the circular stairs now and Jack noticed, as they wended their way round and round the building, that a long steel wire dangled down into the well of the stairway from the very top of the tower. On the end of the wire was a heavy weight. Of course the boys sought a reason for this, and when they reached the lower light room where Mr. Warner found his goggles, Jack asked him about it.
“That weight,” said the engineer as he adjusted the amber glasses, “operates the clock works in the lamp, which in turn drive the lenses round and round the lamp and produce the flash. Come aloft now and I’ll explain it all in detail, only first put on your glasses because your eyes will not stand the strain of looking into the light.”
The boys did as requested and a moment later they climbed up the last section of the spiral stairs and entered the light. This was a platform on the very top of the tower enclosed by eight panels of glass. There in the center, revolving slowly round and round an incandescent oil vapor lamp was the big lens. Mr. Warner began to explain immediately.
“To begin with, the first forms of light were, of course, wood and coal fires which were burned in braziers. These lights were used in England and in the Mediterranean for a long time. The next step was naturally the candle, but no matter how big they made their tallows, or how many of them they crowded into a lantern, the result was a very weak light. Then came the oil lamps of all varieties. Some burned one kind of oil, some another; a few had one kind of a wick, a few had another type, and so on. Indeed, the experimenting with marine lights has extended over a long period and even to-day there is no universal form of lighting for lighthouses. But that, of course, is because conditions are not the same at each light. Acetylene gas is used for light buoys and similar purposes, and electricity is used where a supply is available, or where it can be manufactured conveniently; but the most satisfactory illuminant, all things considered, is kerosene oil. Indeed, the Lighthouse Service consumes more than half a million gallons of kerosene annually.
“For a long time lamps with from one to five concentric wicks were used in the majority of lighthouses, but these are gradually giving way to incandescent oil lamps, such as the one you see behind the lens there. It is a small compact affair and it gives a most brilliant light and at the same time consumes very little oil. The kerosene, which is supplied from a tank in the lower light room, is heated and vaporized, the vapor mixing with air under an incandescent mantle and burning as steadily as an incandescent lamp in a city street, only brighter.”
“That’s very interesting,” said Ray, who had been watching the lamp for some time. “Now tell us something about the lens, won’t you? How did they come to invent such a complicated-looking affair?”
“Yes, I’ll tell you about the lens. Old-time lighthouse engineers were always experimenting on how to improve the efficiency of a light and when they got through changing the forms of fuel they tried the use of reflectors of various types. Their efforts were more or less successful, but when a French physician by the name of Fresnel came forward with an elaborate system of lenses the science of coast lighting was revolutionized. This lens you see before you is the present-day result of his efforts. It embodies his idea worked out to perfection. You will notice that there is a central lens, or bull’s-eye, and that around it are grouped prisms of highly polished glass. The idea is this: The light throws rays on every side, back, front, top, bottom, and all over. Well, these prisms of glass grasp, as it were, each ray that shoots out at the side and top and literally bend it and shoot it forward. In that way all the light from the lamp is gathered into one bundle and sent out in a given direction, instead of radiating off on all sides. The lens works exactly like a megaphone which your football rooters use at Drueryville, Jack. Do you get the idea?”
“Indeed I do and it is mighty interesting,” assured the young Vermonter.
“Good, and now if you’ve seen all you want up here we’ll go down in the lower light room, for it is hot in the lantern here and besides even with these goggles the bright light hurts my eyes.”
“Mine too,” said Ray, leading the way through the tiny trap door and down the stairway to the lower light room.