which would burn, and that might mean a
catastrophe for us."

"Then we are certainly stopped at this cave."

"Not by any means," rejoined John, and he saw the boys' faces smiling
again. "We must make a safety lamp."

"Do you mean a lamp that will not explode the gas, although it has
enough oxygen to 'support combustion,' as you say?"

"Exactly. Have you ever heard of the Humphrey Davy lamp? Well, that was
invented to meet the very condition found here."

"Tell us about it."

"In 1816 Davy discovered that a flame would not pass through a fine wire
gauze, while conducting some experiments. It occurred to him that use
could be made of this discovery by surrounding the flame of a lamp with
gauze, and the well-known lamp was built on that principle."

"What I am curious to know is, that if it will not burn the gas, will it
go out if it gets down in the gas?"

[Illustration: _Fig. 13. Types of Safety Lamps._]

"Yes; because enough air, or oxygen must go through the mesh to support
combustion of the flame itself. If it does not get enough it is
smothered."

"Then why not make a lamp of that kind?"

"But where can we get enough gauze to make a cylinder big enough to go
around a lamp?"

John laughed at the serious predicament, which expressed itself in the
faces of the boys. "That is true," he said; "but if we can get a small
piece of tin, we can punch it full of fine holes, and probably make
that answer."

"We haven't anything in the way of tin large enough to go around a lamp,
but here is a round piece, about three inches in diameter."

"That will answer; punch that as full of holes as possible, and be sure
they are very small."

"What shall we use for a lamp?"

John was already looking around, and soon spied a tree in the distance
that looked like a small pine, and beneath that he found some cones, a
dozen of which were picked up.

[Illustration: _Fig 14. How John made the Lamp._
_A. Perforated Disk_
_B. Frame of Sticks_
_C. Cloth Covering_
_D. Base Plate._]

"That is a pine tree, isn't it?"

"Yes; these cones will burn for some time."

"But they will not make much of a light."

"No; but we are not after a light, but they will do for testing
purposes."

The accompanying sketches show how it was made. A plate was used for a
base, on which the burning cone was placed. A half dozen twigs were
then provided, and these were bent U-shaped, after being secured
together at their middle portions, a