“I trust it will be so in this case,” said Tom.

“So do I,” said Jack hopefully, but a cold fear that his father was ruined possessed him, and made his heart feel heavy as lead.

Suddenly, from the purple firmament, came the sound of distant thunder.  Following it a puff of wind, hot as the exhalation of an opened oven, blew in their faces.  In the distance they saw a ragged streak of lightning tear the cloud curtains.

CHAPTER XIX.

The “Lightning cage.”

“Look at that, will you!” 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.