"All three of us," said he, "do not weigh much more
than four hundred pounds."

"But, sir," said Joe, "if it was necessary for your
expedition, I could make myself thinner by twenty pounds,
by not eating so much."

"Useless, my boy!" replied the doctor. "You may
eat as much as you like, and here's half-a-crown to buy
you the ballast."



CHAPTER SEVENTH.

Geometrical Details.--Calculation of the Capacity of the Balloon.--The
Double Receptacle.--The Covering.--The Car.--The Mysterious Apparatus.
--The Provisions and Stores.--The Final Summing up.

Dr. Ferguson had long been engaged upon the details
of his expedition. It is easy to comprehend that the balloon
--that marvellous vehicle which was to convey him
through the air--was the constant object of his solicitude.

At the outset, in order not to give the balloon too
ponderous dimensions, he had decided to fill it with
hydrogen gas, which is fourteen and a half times lighter
than common air. The production of this gas is easy,
and it has given the greatest satisfaction hitherto in