"If it doesn't rain, Bill, tomorrow will be Monday. However, Dr. Bert (Professor in the Paris Faculty of Sciences) proved 'that the lessening of the barometric pressure,' to use his own words, 'is of no account, mechanically, in the production of the phenomena.' Yes, he proved that, to use his own words again, 'it is not the lowering of mechanical pressure that produces the symptoms, but the low tension prevents the oxygen from entering the blood in sufficient quantity.' "Dr. Bert not only experimented on sparrows but entered the air-chamber himself. As the pressure was reduced, he experienced all the symptoms of mountain sickness. "'But,' he says, 'all these symptoms disappeared as by enchantment as soon as I respired some of the oxygen in the bag; returning, however, when I again breathed the air of the cylinder.' "In one of his experiments, the pressure was reduced to 246 millimeters--9.7 inches. "'This,' he says, 'is exactly the pressure on the highest summit of the Himalayas--the same degree of pressure which was so near proving fatal to Glaisher and Coxwell; I reached this point without the slightest sense of discomfort, or, to speak more accurately, the unpleasant sensations I felt at the beginning had entirely disappeared. A bird in the cylinder with me was leaning on one side, and very sick. It was my wish to continue the experiment till the bird died, but the steam-pump, conspiring, as I suspect, with the people who were watching me through glass peep-holes, would not work, and so I had to return to normal pressure.' "So you see, Bill, it is the low tension of the oxygen and not the diminished pressure that produces the distress and suffering and even death." "All this is very interesting, but our problem is not one of rarefied air; the atmosphere _here_ is _compressed_." "And, in compressed air," said Milton Rhodes, "it is the oxygen again that produces the symptoms. Subject a sparrow to a pressure of twenty atmospheres, and the bird is thrown into convulsions, stronger than those produced by tetanus or strychnine, convulsions which soon end in death. If pure oxygen is used, a pressure of only five atmospheres kills the sparrow. But, and mark this, if the air be deficient in oxygen, the pressure of twenty atmospheres does not produce even a tremor. "So you see, Bill," Milton concluded, "we could descend to a very great depth in an atmosphere poor in oxygen." "But how do we know that the atmosphere down there is poor in oxygen? It may be nothing of the kind. It may be saturated with it."