There were two different kinds of waves produced in the air by the tremendous forces at work in the eruption of Krakatoa; namely, atmospheric waves and sound waves.
The atmospheric waves showed their presence in the air by means of changes produced in the atmospheric pressure. Now, while these changes cannot readily be felt by man, yet their presence can be easily shown by the use of instruments called barometers.
There are in different parts of the world, buildings called meteorological observatories, that are provided, among other instruments, with recording barometers. These instruments caught the great atmospheric waves that were produced by the eruption of Krakatoa. In this manner, the astounding fact was learned that the waves starting from the volcano travelled no less than seven times around the world. When we say astounding, it must not be understood that the formation of such waves was at all contrary to the known laws of physics. On the contrary, provided the force of the eruption was sufficiently great, such waves must have been produced in the great ærial ocean. The astonishing, or wonderful thing, was that the force setting up these waves was so great that it caused them to move seven times around the globe.
The atmospheric waves were so powerful that it will be worth our while to describe them in detail. Starting from the volcano of Krakatoa, as a centre, these waves moved outwards in all directions, becoming gradually larger and larger until they reached a point halfway round the globe, or 180° from Krakatoa. The waves did not, however, stop here, but continued moving onward, now growing smaller and smaller until they reached a point in North America, immediately opposite Krakatoa. Such a point on a globe is called an antipodal point.[1] The waves did not stop at this point, but again advanced moving toward Krakatoa, growing larger and larger until they again reached a point halfway around the globe, or 180° from Krakatoa, when they again continued moving but now continually growing smaller and smaller, until they reached Krakatoa. Here they again began moving completely around the globe, and this was continued for as many as seven times. It must not be supposed that the waves ceased on the seventh time around. On the contrary, they, probably, kept on moving for many additional times, but they were then so feeble that even the sensitive recording barometers were unable to detect their presence.
There was another kind of waves in the atmosphere that did not require barometers for their detection. These were the sound waves, and can readily be detected by the human ear.
Now, in the case of the great eruption of Krakatoa, the intensity of the sounds was so great that the sounds could be heard distinctly at distances of several thousand miles from Krakatoa.
The sound waves so closely resembled the explosion of artillery that at Acheen, a port on the northern coast of Sumatra, 1,073 miles from Krakatoa, the authorities, believing that an attack was being made on the port, placed all their troops under arms to repel the invaders. The sounds were also distinctly heard at Bangkok, in Siam, a distance of 1,413 miles from the volcano. They were also heard at the Chagos Islands, a group of islands situated in the Indian Ocean about 2,267 miles from Krakatoa.
Two steamers at Singapore, 522 miles distant, were despatched to find the vessel that was believed to be firing guns as distress signals.
The sounds were distinctly heard in parts of South Australia, 2,100 miles distant, and in Western Australia, at 1,700 miles distance.
But it will be unnecessary to give any further details of the great distances at which these sounds were actually heard. It will suffice to say that they were heard as far off as about 3,000 miles.