The air about them seemed to rock with the shock, there was a dull roaring sound which hummed continually in their ears, and the vibrations of the earth continued. They were indeed experiencing a most serious earthquake. CHAPTER X THE BLACK DAY The 'quake was over in a very few moments; the Indians and Washington White, however, cowered upon the ground for some time, crying out their fear of what they considered supernatural phenomena. Jack Darrow and Mark Sampson were not frightened in the same way as the darkey and the Aleuts; nevertheless they were much shaken. Professor Henderson, however, displayed naught but the keenest interest in the scientific side of the happening. He clambered to his feet the moment he could stand, and observed: "A most pronounced seismic disturbance--I should say earthquake." "I should say it was pronounced!" grunted Phineas Roebach. Being a fat man, he had fallen heavily. He was now rubbing himself tenderly where he had been bruised upon the hard ground. "This shock beats the one we had the other day." "Not a shock, my dear sir," said Professor Henderson, quickly. "An earthquake is not, strictly speaking, a shock at all. Within the past twenty years science has learned to measure and to study earthquakes. If we have learned nothing else, we have learned that an earthquake is _not_ a shock." "It tumbled us about a whole lot, then, Professor," said Jack Darrow. "What would you call it, if not a shock?" The phenomena being over for the time--as all could see--they returned to the cabin to complete their meal. Roebach had said something soothing to his Indians, but they, like Washington White, preferred remaining in the open. Wash sat down beside the cage of his pet rooster, and declared to the boys when they urged him to come in again: "No, sah! I ain't hongry, nohow. An' w'edder de professor am right dat dese yer earthquakes ain't shockin', I kin tell yo' right now dat it shocked _me_! Nor I ain't gwine ter gib it no secon' chance ter tumble dat ruff down on ma haid--no, sah!" Once more at the breakfast table, with the affrighted Indian squaw waiting upon them, the professor took up the topic of earthquakes again, in answer to Jack's observation. "From the time of the ancients to the middle of the last century the phenomena of earthquakes were observed and described upon countless occasions," he said. "Yet even Humboldt's 'Cosmos', published as late as 1844, which summarized the then existing knowledge on the subject, did not suggest that earthquakes should be studied like other mechanical motions.