wly and went as straight as
the string of a plumb-line. The sun was bisected by the line of the
horizon, and appeared to be moving about them in a circle, with only
its upper half visible. As Jupiter's northern hemisphere was passing
through its autumnal equinox, they concluded they had landed exactly at
the pole.

"Now to work on our experiment," said Cortlandt. "I wonder how we may
best get below the frozen surface?"

"We can explode a small quantity of dynamite," replied Bearwarden,
"after which the digging will be comparatively easy."

While Cortlandt and Bearwarden prepared the mine, Ayrault brought out a
pickaxe, two shovels, and the battery and wires with which to ignite
the explosive. They made their preparations within one hundred feet of
the Callisto, or much nearer than an equivalent amount of gunpowder
could have been discharged.

"This recalls an old laboratory experiment, or rather lecture," said
Cortlandt, as they completed the arrangements, "for the illustration is
not as a rule carried out. Explode two pounds of powder on an iron
safe in a room with the windows closed, and the windows will be blown
out, while the safe remains uninjured. Explode an equivalent amount of
dynamite on top of the safe, and it will be destroyed, while the glass
panes are not even cracked. This illustrates the difference in
rapidity with which the explosions take place. To the intensely rapid
action of dynamite the air affords as much resistance as a solid
substance, while the explosion of the powder is so slow that the air
has time to move away; hence the destruction of the windows in the
first case, and the safe in the second."

When they had moved beyond the danger line, Bearwarden, as the party's
practising engineer, pressed the button, and the explosion did the
rest. They found that the ground was frozen to a depth of but little
more than a foot, below which it became perceptibly warm. Plying their
shovels vigorously, they had soon dug the hole so deep that its edges
were above their heads. When the floor was ten feet below the
surrounding level the thermometer registered sixty.

"This is scarcely a fair test," said Cortlandt, "since the heat rises
and is lost as fast as given off. Let us therefore close the opening
and see in what time it will melt a number of cubic feet of ice."

Accordingly they climbed out, threw in about a cart-load of ice, and
covered the opening with two of the Callisto's thick ru