who was accustomed
always in lecturing to watch the countenance of the least intelligent
of his audience, and when he felt that he had made his meaning clear to
him, he concluded that he must have succeeded with all the rest. In
this case, however, it was technical ignorance, rather than any lack
of intelligence, that justified the selection of the orderly for this
special attention.

Satisfied with his scrutiny of Ben Zoof's face, the professor went on.
"And now, gentlemen, we have to see what these coins weigh here upon
Gallia."

He suspended the money bag to the hook; the needle oscillated, and
stopped. "Read it off!" he said.

The weight registered was one hundred and thirty-three grammes.

"There, gentlemen, one hundred and thirty-three grammes! Less than
one-seventh of a kilogramme! You see, consequently, that the force of
gravity here on Gallia is not one-seventh of what it is upon the earth!"

"Interesting!" cried Servadac, "most interesting! But let us go on and
compute the mass."

"No, captain, the density first," said Rosette.

"Certainly," said the lieutenant; "for, as we already know the volume,
we can determine the mass as soon as we have ascertained the density."

The professor took up the cube of rock. "You know what this is," he went
on to say. "You know, gentlemen, that this block is a cube hewn from
the substance of which everywhere, all throughout your voyage of
circumnavigation, you found Gallia to be composed--a substance to which
your geological attainments did not suffice to assign a name."

"Our curiosity will be gratified," said Servadac, "if you will enlighten
our ignorance."

But Rosette did not take the slightest notice of the interruption.

"A substance it is which no doubt constitutes the sole material of
the comet, extending from its surface to its innermost depths. The
probability is that it would be so; your experience confirms that
probability: you have found no trace of any other substance. Of this
rock here is a solid decimeter; let us get at its weight, and we shall
have the key which will unlock the problem of the whole weight of
Gallia. We have demonstrated that the force of attraction here is only
one-seventh of what it is upon the earth, and shall consequently have to
multiply the apparent weight of our cube by seven, in order to ascertain
its proper weight. Do you understand me, goggle-eyes?"

This was addressed to Ben Zoof, who was staring hard at him. "No!" said