han four thousand years, when it was well known by
the Egyptians.

[Illustration: _Fig. 30. Forming blade for inside of Crock._]

Since the day that the boys visited Observation Hill, at the time they
discovered the skeleton and fragments of the boat, no attempt had been
made to visit the cave. That was the mission when they accidentally made
their surprising discovery.

George did not, however, feel that they should again make the attempt
until they had a better lighting means than the unsatisfactory candles,
and when the supposed petroleum vein dashed all hopes of lighting
material from that quarter, the only remedy seemed to be by way of
improving their candle-light.

Harry had progressed well in the making of the battery. It was now in a
completed state, and he announced that the first tests would be made the
next day. In the morning all assembled in the factory, and the sulphuric
acid solution was made up.

The Professor inspected the batteries. Ten cells had been prepared, so
that they could have, at least, fifteen volts. When all the cells had
been connected together--that is, the positive pole of one cell with the
negative pole of the other--a wire was attached at each end of the row
of cells, at the last electrodes, so as to form the outside connections.

When the two outside wires were brought together and their contact
broken a spark was plainly shown, which was an indication that the
battery was generating electricity. The boys danced about with joy at
this exhibition. From that time forward the battery was one of the most
interesting things in the laboratory, and what they finally accomplished
with it will be fully detailed as we go along in their history.

George thought he saw a way to make the light necessary for the cave.
"Why can't we rig up an electric light now and explore the cave?"

"We might do that, but we have several things to do before we can have
light from that source."

"Haven't we the electricity for it?"

"Do you know how many years electricity was known before electric light
was discovered? Before we can utilize this agency for lighting purposes,
we must make a machine which will produce a vacuum; we must make glass;
we must learn to carbonize threads; and the art of blowing glass would
be a necessary accomplishment."

"As usual," said Harry, "something must be made to make something that
makes something else."

"But can we make electric light without putting it in a glass