Frank asked the Doctor what was the cause of an earthquake, and whether the movements of the ground were always the same. "The cause is difficult to get at," Doctor Bronson answered, "although the wisest men in all ages have studied the phenomenon, and endeavored to make a satisfactory theory for it. The ancient philosophers supposed that the winds became imprisoned in the earth, and, in their struggles to escape, gave rise to the upheaval of the land and the general convulsions. In the last century a French scientist contended that large quantities of bituminous and sulphurous matter became suddenly inflamed, and broke forth in violent fermentations. The still more modern theory is that the surface of the earth is only a thin crust over a mass of melted matter; that we live on the outside of a ball of liquid fire, which is liable to explode at any time, and put an end to us and all around. It is argued that the volcanoes now in activity are the outlets for this internal fire, and the occasional eruptions and earthquakes are the result of the fire seeking vent, in consequence of the clogging of the subterranean passages." Frank thought the theory was not very comforting, and on the whole he preferred the older one. Fred agreed with him, and then the Doctor continued: "The movements of the ground are not always the same in earthquakes; sometimes there will be only a single shock, or more frequently two about twenty seconds or even a minute apart, and again there will be a succession of shocks lasting for hours, or even days, at irregular intervals. The latter is oftener the case in South America than in other parts of the world; there the ground keeps up a rocking more or less continual for hours, and the later shocks are generally more violent than the earlier ones. Usually, however, the earthquake comes without warning, and is over in less than a minute, but in that space of time the destruction may be terrible. [Illustration: DESTRUCTION OF MESSINA IN 1783.]