"Local attraction, Bill. Many instances of this could be given. One will suffice. Lieutenant Underwood, of the Wilkes Exploring Expedition, found a deviation of thirteen and a quarter points on the summit of the Cobu Rock, in the Feejees—one hundred and forty-nine degrees. The Island of Nairai was directly north, and yet, according to the compass, it bore southeast-by-south one quarter south, whilst, placed at the foot of the rock, that very same compass said Nairai bore north! So you see that that faithful friend of man, and especially of the mariner, has in its friendships some qualities that are remarkably human.
"Still," Rhodes added, "I wish that we had brought one along. Also, we should have brought a manometer, for the aneroid will be worthless after we have descended below sea-level. Oh, well, the boiling-point of water will give us the atmospheric pressure: under a pressure of two atmospheres, water boils at 249.5° Fahrenheit; under a pressure of three atmospheres, at 273.3°; four atmospheres, 291.2°; five, 306°; six, 318.2°; seven, 329.6°; eight, 339.5°; and so on. On the summit of Rainier, it boils at about 185°."
"I wish that we were headed for the summit," said I. "Eight atmospheres! When we reach that pressure—if we ever do—we'll be ten and a half miles below the level of the sea, won't we?"
Rhodes nodded.
"According to the law. But, as I remarked, there is something wrong with the law. 'Tis my belief that we shall be able to descend much deeper than ten and one half miles—that is, that the atmospheric pressure will permit us to do so."
"That qualification," I told him, "is very apropos, for there is no telling what the inhabitants of this underground world will permit us to do or will do to us—bat-apes or ape-bats, humans, or both."
"That, of course, is very true, Bill."
"And," said I, "we won't need a manometer, or we won't need to ascertain the boiling-point of water, to know that the pressure is increasing. Our eardrums will make us painfully aware of that fact."
"When that comes, swallow, Billy, swallow, and the pain will be no more."
"Swallow?"