It seems to be certain that if all the materials poured forth from Krakatoa during the critical period could be collected together, the mass they would form would be considerably over a cubic mile in volume. It is in the other standards of comparison that the importance of the explosion of Krakatoa is to be sought. The intensity of this outbreak in its last throes was such that mighty sounds were heard and mighty waves arose in the sea for which we can find no parallel. Every part of our globe’s surface felt the pulse of the air-waves, and beautiful optical phenomena made the circuit of the globe even more than once or twice. In these last respects the eruption of Krakatoa is unique. It appears to me that the most remarkable incident connected with the eruption of Krakatoa was the production of the great air-wave by that particular explosion that occurred at ten o’clock on the morning of Monday, August 27. The great air-wave was truly of cosmical importance, affecting as it did every particle of the atmosphere on our globe. The comprehensive series of phenomena wherein the atmosphere of the entire globe participates in an organized vibration has, so far as we know, only once been witnessed, and that was after the greatest outbreak at Krakatoa, at ten o’clock on the morning of August 27. But the ebb and the flow of these mighty undulations are not immediately appreciable to the senses. The great wave, for instance, passed and re-passed and passed again over London, and no inhabitant was conscious of the fact. But the automatic records of the barometer at Greenwich show that the vibration from Krakatoa to its antipodes, and from the antipodes back to Krakatoa, was distinctly perceptible over London not less than six or seven times. From all parts of Europe, from Berlin to Palermo, from St. Petersburg to Valencia, we obtain the same indications. Fortunately self-recording barometric instruments are now to be found all over the world. Almost all the instruments show distinctly the first great wave from Krakatoa to its antipodes in Central America, and the return wave from the antipodes to Krakatoa. They also all show the second great wave which sped from Krakatoa, as well as the second great wave which returned from the antipodes. Thus, the first four of the oscillations are depicted on upward of forty of the barograms. The fifth and sixth oscillations are also to be distinguished on several of the curves, and even the seventh is certainly established at some few places, of which Kew is one. Then the gradually increasing faintness of the indications renders them unrecognizable, from which we conclude that after seven pulsations our atmosphere had sensibly regained its former condition ere it was disturbed by Krakatoa.