“There are almost as many conjectures as to that as there are geologists,” was his answer.  “One estimates it thirty miles, because the internal heat, increasing at the rate of about one degree to each sixty to seventy feet depth, would be sufficient to fuse the most refractory substances at that distance beneath the surface.  Another finds that the phenomena of precession and nutation require that the earth, if not entirely solid, must at least have a shell not less than eight hundred to a thousand miles in thickness.  So there you are.  You may take your choice.”

“And if it should prove solid?” I asked.

“It will be all the same to us in the end, David,” replied Perry.  “At the best our fuel will suffice to carry us but three or four days, while our atmosphere cannot last to exceed three.  Neither, then, is sufficient to bear us in the safety through eight thousand miles of rock to the antipodes.”

“If the crust is of sufficient thickness we shall come to a final stop between six and seven hundred miles beneath the earth’s surface; but during the last hundred and fifty miles of our journey we shall be corpses.  Am I correct?” I asked.

“Quite correct, David.  Are you frightened?”

“I do not know.  It all has come so suddenly that I scarce believe that either of us realizes the real terrors of our position.  I feel that I should be reduced to panic; but yet I am not.  I imagine that the shock has been so great as to partially stun our sensibilities.”

Again I turned to the thermometer.  The mercury was rising with less rapidity.  It was now but 140 degrees, although we had penetrated to a depth of nearly four miles.  I told Perry, and he smiled.

“We have shattered one theory at least,” was his only comment, and then he returned to his self-assumed occupation of fluently cursing the steering wheel.  I once heard a pirate swear, but his best efforts would have seemed like those of a tyro alongside of Perry’s masterful and scientific imprecations.

Once more I tried my hand at the wheel, but I might as well have essayed to swing the earth itself.  At my suggestion Perry stopped the generator, and as we came to rest I again threw all my strength into a supreme effort to move the thing even a hair’s breadth—­but the results were as barren as when we had been traveling at top speed.

I shook my head sadly, and motioned to the starting lever.  Perry pulled it toward him, and once again we were plunging downward toward eternity at the rate of seven miles an hour.  I sat with my eyes glued to the thermometer and the distance meter.  The mercury was rising very slowly now, though even at 145 degrees it was almost unbearable within the narrow confines of our metal prison.

About noon, or twelve hours after our start upon this unfortunate journey, we had bored to a depth of eighty-four miles, at which point the mercury registered 153 degrees F.