Only, never fear, you won't be expected to make yourself lean, if you're found to be heavy. You'll go as you are."

"Well, I can tell you, I am not going to let myself be weighed," said Kennedy, firmly.

"But, sir, it seems that the doctor's machine requires it."

"Well, his machine will have to do without it."

"Humph! and suppose that it couldn't go up, then?"

"Egad! that's all I want!"

"Come! come, Mr. Kennedy! My master will be sending for us directly."

"I shan't go."

"Oh! now, you won't vex the doctor in that way!"

"Aye! that I will."

"Well!" said Joe with a laugh, "you say that because he's not here; but when he says to your face, 'Dick!' (with all respect to you, sir,) 'Dick, I want to know exactly how much you weigh,' you'll go, I warrant it."

"No, I will NOT go!"

At this moment the doctor entered his study, where this discussion had been taking place; and, as he came in, cast a glance at Kennedy, who did not feel altogether at his ease.

"Dick," said the doctor, "come with Joe; I want to know how much you both weigh."

"But--"

"You may keep your hat on. Come!" And Kennedy went.

They repaired in company to the workshop of the Messrs. Mitchell, where one of those so-called "Roman" scales was in readiness. It was necessary, by the way, for the doctor to know the weight of his companions, so as to fix the equilibrium of his balloon; so he made Dick get up on the platform of the scales. The latter, without making any resistance, said, in an undertone:

"Oh! well, that doesn't bind me to any thing."

"One hundred and fifty-three pounds," said the doctor, noting it down on his tablets.

"Am I too heavy?"

"Why, no, Mr. Kennedy!" said Joe; "and then, you know, I am light to make up for it."

So saying, Joe, with enthusiasm, took his place on the scales, and very nearly upset them in his ready haste. He struck the attitude of Wellington where he is made to ape Achilles, at Hyde-Park entrance, and was superb in it, without the shield.

"One hundred and twenty pounds," wrote the doctor.

"Ah! ha!" said Joe, with a smile of satisfaction And why did he smile? He never could tell himself.

"It's my turn now," said Ferguson--and he put down one hundred and thirty-five pounds to his own account.

"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 aerostatic experiments.

The doctor, according to very accurate calculations, found that, including the articles indispensable to his journey and his apparatus, he should have to carry a weight of 4,000 pounds; therefore he had to find out what would be the ascensional force of a balloon capable of raising such a weight, and, consequently, what would be its capacity.

A weight of four thousand pounds is represented by a displacement of the air amounting to forty-four thousand eight hundred and forty-seven cubic feet; or, in other words, forty-four thousand eight hundred and forty-seven cubic feet of air weigh about four thousand pounds.

By giving the balloon these cubic dimensions, and filling it with hydrogen gas, instead of common air--the former being fourteen and a half times lighter and weighing therefore only two hundred and seventy-six pounds--a difference of three thousand seven hundred and twenty-four pounds in equilibrium is produced; and it is this difference between the weight of the gas contained in the balloon and the weight of the surrounding atmosphere that constitutes the ascensional force of the former.