boards," said the Irishman;
"look here!" So saying, he leaped from the yard and revolved in the
air at least twenty times before alighting on the deck.

"Now," said the professor, "I'll explain why you only weigh seventeen
pounds as indicated by the spring-balance. We have sailed, down the
gulf 500 miles, haven't we?"

"Yis, sorr."

"And here we are sailing upside down on the inside roof of the
world----"

"Sailin' upside down? Indeed, sorr, an' ye can't make me believe that,
for shure I'm shtandin' on me feet like yourself, head uppermost."

"Well, whether you believe it or not, we are sailing upside down, just
as ships going to Australia sail upside down as compared with ships
sailing the North Atlantic. But the point of gravity is this: Here we
are surrounded on all sides by the shell of the earth, which attracts
equally in all directions. Hence all objects in the interior world
have no weight as regards whatever thickness of the earth's shell
surrounds them. You see, weight is caused by an object having the
world on one side of it. Thus both the world and the object attract
each other according to the density and distance apart. What we call a
pound weight is a mass of matter attracted by the earth on its surface
with a force equal to the weight of sixteen ounces. A pound weight on
the surface of the earth weighs sixteen ounces, and all the mighty
volume of our planet, with all its mountains, continents and seas,
weighs only sixteen ounces on the surface of a pound weight. The earth
may still weigh many millions of tons as regards the sun, but as
regards a pound weight it only weighs sixteen ounces."

"That is an illustration of Flathootly's mental calibre," said Captain
Wallace. "He only believes what his brain can accommodate in the way
of knowledge."

"God bless the captain," said Flathootly, "I'm shure his brain is as
big as mine any day in the week."

"Now," continued the astronomer, "it seems to me that the substances
of the earth, rocks, metals, and water, have, under the influence of
centrifugal gravity, massed themselves very thickly at the equator or
point of greatest motion, and stretch toward the poles in a
gradually-lessening mass until the polar gulfs are reached. Thus the
earth's shell resembles a musk-melon with the inside cleaned out."

"It makes me mouth wather to think of it," said Flathootly.

"Now, listen," said the astronomer; "we are also under the influence
of the earth's centri