u!" exclaimed Tom.

"What, you are not scared, are you?" asked Jack.

"N-no, but I must say I'm not fond of thunderstorms Particularly when
we are carrying all that gas over our heads."

"That new invention of mine will take care of that all right," said
Jack confidently.

He referred to a new device of his with which the Wondership was
equipped for protecting balloon bags from lightning. In a thunderstorm
a balloon, or gas-filled dirigible, is subject to sudden variations of
electric charge which, under certain conditions, might produce sparks
leading to its annihilation.

More especially was this the case with such a craft as this
Wondership, carrying as she did so much metal and steel wiring. The
netting of the bag, with the idea of making it as conductive as
possible, was of metal, connecting with the other metal parts of the
craft so that when a steel drag rope was lowered to the ground a
discharge of lightning striking the balloon would be passed off
harmlessly into the earth, as is the case with a lightning conductor.

It might be supposed that making the outside of a balloon a good
conductor would invite danger from lightning. But the Boy Inventors
knew that this was not the case. While the ordinary balloon envelope
is a fairly good insulator against low voltage, it is unable to resist
the high tension of atmospheric electricity.

Jack ascertained these facts by touching an electroscope with a bit of
balloon cloth of the kind used on the Wondership, and charged with
2,000 volts of electricity. The electroscope instantly responded.

This showed that the balloon bag increased the electrical tension
immediately above and below it as much as it would do if it was a
perfect conductor, but the destructive action of a lightning bolt
would be greater in proportion to the resistance opposed to it. So
that, in reality, Jack's device was one of the safest that could be
imagined for protecting balloonists in a heavy storm.

In effect, the occupants of the Wondership were enclosed in a cage.
Lightning might zip through the wires and stays, but it could not
touch them. As to the danger of letting out gas through the valve in a
strong electric field, which is almost certain to produce sparks, the
boys did not have to worry about that for to deflate the bag they
simply pumped some of its contents back into the reservoir with the
powerful gas pumps.

But after all, Jack's device had never been tested. It looked as if