But no doubt these others answer pressing needs unique to the Nautilus. That dial I see there, with the needle moving across it--isn't it a pressure gauge?"

"It is indeed a pressure gauge. It's placed in contact with the water, and it indicates the outside pressure on our hull, which in turn gives me the depth at which my submersible is sitting."

"And these are some new breed of sounding line?"

"They're thermometric sounding lines that report water temperatures in the different strata."

"And these other instruments, whose functions I can't even guess?"

"Here, professor, I need to give you some background information," Captain Nemo said. "So kindly hear me out."

He fell silent for some moments, then he said:

"There's a powerful, obedient, swift, and effortless force that can be bent to any use and which reigns supreme aboard my vessel. It does everything. It lights me, it warms me, it's the soul of my mechanical equipment. This force is electricity."

"Electricity!" I exclaimed in some surprise.

"Yes, sir."

"But, captain, you have a tremendous speed of movement that doesn't square with the strength of electricity. Until now, its dynamic potential has remained quite limited, capable of producing only small amounts of power!"

"Professor," Captain Nemo replied, "my electricity isn't the run-of-the-mill variety, and with your permission, I'll leave it at that."

"I won't insist, sir, and I'll rest content with simply being flabbergasted at your results. I would ask one question, however, which you needn't answer if it's indiscreet. The electric cells you use to generate this marvelous force must be depleted very quickly. Their zinc component, for example: how do you replace it, since you no longer stay in contact with the shore?"

"That question deserves an answer," Captain Nemo replied. "First off, I'll mention that at the bottom of the sea there exist veins of zinc, iron, silver, and gold whose mining would quite certainly be feasible. But I've tapped none of these land-based metals, and I wanted to make demands only on the sea itself for the sources of my electricity."

"The sea itself?"

"Yes, professor, and there was no shortage of such sources. In fact, by establishing a circuit between two wires immersed to different depths, I'd be able to obtain electricity through the diverging temperatures they experience; but I preferred to use a more practical procedure."

"And that is?"

"You're familiar with the composition of salt water. In 1,000 grams one finds 96.5% water and about 2.66% sodium chloride; then small quantities of magnesium chloride, potassium chloride, magnesium bromide, sulfate of magnesia, calcium sulfate, and calcium carbonate. Hence you observe that sodium chloride is encountered there in significant proportions. Now then, it's this sodium that I extract from salt water and with which I compose my electric cells."

"Sodium?"

"Yes, sir. Mixed with mercury, it forms an amalgam that takes the place of zinc in Bunsen cells. The mercury is never depleted. Only the sodium is consumed, and the sea itself gives me that. Beyond this, I'll mention that sodium batteries have been found to generate the greater energy, and their electro-motor strength is twice that of zinc batteries."

"Captain, I fully understand the excellence of sodium under the conditions in which you're placed. The sea contains it. Fine. But it still has to be produced, in short, extracted. And how do you accomplish this? Obviously your batteries could do the extracting; but if I'm not mistaken, the consumption of sodium needed by your electric equipment would be greater than the quantity you'd extract. It would come about, then, that in the process of producing your sodium, you'd use up more than you'd make!"

"Accordingly, professor, I don't extract it with batteries; quite simply, I utilize the heat of coal from the earth."

"From the earth?" I said, my voice going up on the word.

"We'll say coal from the seafloor, if you prefer," Captain Nemo replied.