An opportunity of observing this speed of sound waves on a gigantic scale occurred about 20 years ago on the occasion of a great volcanic eruption near Java. If you open the map of Asia and look for Java and Sumatra in the Asiatic Archipelago, you will easily find the Sunda Strait, and on a good map you will see a small island marked called Krakatoa. This island possesses, or rather did possess, a volcano which, until the year 1883, had not been known to be in eruption. In that year, however, it again burst into activity, and after preliminary warnings a final stupendous outburst occurred on August 27, 1883. The roar of this volcanic explosion was probably the loudest noise ever heard upon this earth. The pent-up volcanic gases and vapours burst forth from some subterranean prison with such appalling power that they created an air wave which not only encircled the earth, but reverberated to and fro seven times before it finally faded away. The zone of compressed air forming the mighty air wave as it passed from point to point on the earth’s surface, caused an increase of atmospheric pressure which left its record on all the self-registering barometers, and thus enabled its steps to be traced. A diligent examination of these records, as collected in a celebrated Report of the Royal Society upon the Eruption of Krakatoa, showed exactly the manner in which this great air wave expanded. Starting from Krakatoa at 10 a.m. on the 27th of August, 1883, the air wave sped outwards in a circle of ever-increasing diameter until, by 7 p.m. on the same day, or 9 hours later, it formed a girdle embracing the whole world. This stupendous circular air wave, 24,000 miles in circumference, then contracted again, and in 9 hours more had condensed itself at a point in the northern region of South America, which is the antipodes of Krakatoa. It then rebounded, and, expanding once more, just like a water wave reflected from the side of a circular trough, returned on its own steps, so that 36 hours afterwards it had again reached the point from whence it set out. Again and again it performed the same double journey, but each time weaker than before, until, after seven times, the echoes of this mighty air wave had completely died away. This is no fancy picture, but a sober record of fact obtained from the infallible records of self-registering air-pressure-measuring instruments. But we have evidence that the actual sound of the explosion was heard, 4 hours after it happened, on the other side of the Indian Ocean, by human ears, and we have in this an instance of the measurement of the velocity of sound on the largest scale on which it was ever made. There are many curious and interesting facts connected with the transmission of a sound wave through air, affecting the distance at which sounds can be heard. The speed of sound in air is much influenced by the temperature of the air and by wind.