Another way of observing these curious little movements is by burying microphones in the ground. The microphone is a little instrument invented of late years which is capable of enormously magnifying the very slightest sounds, such as our ears will not detect. By its means one can hear, as some one said, "the tramp of a fly's foot," if he will be so obliging as to walk over it. It has thus been proved in Italy that the earth sends forth a confused medley of sounds caused by little crackings and snappings in the rocks below our feet. In this way it will be possible to predict a serious earthquake, because it will give warning some days before, by the increase of the little tremors and sounds; and it is to be hoped that by this simple means human lives may be saved. Now, these disturbances are of precisely the same nature as earthquakes,--in fact, we may call them microscopic earthquakes. To the geologist they are of great interest, as they seem to afford some little insight into the difficult question of the upheaval of mountains, and to show us something of the constant _working_ of those wonderful forces below the surface of the earth by means of which continents are raised up out of the sea, and mountain-chains are elevated thousands of feet. It is probable that both are due to the working of the same forces, and are accomplished by the same machinery. We now pass on to consider those more violent movements of the solid land known as earthquakes. This kind of disturbance is such as might be produced by a sudden shock or blow given below the ground, from which waves travel in all directions. First comes a rumbling noise like the roar of distant artillery; then come the earthquake waves one after another, causing the ground to rise and fall as a ship does on the waves of the sea; the ground is frequently rent asunder, so that chasms are formed, into which in some cases men and animals have been hurled alive. In the case of a very violent earthquake the waves travel long distances. Thus the great earthquake by which Lisbon was destroyed in the year 1755 disturbed the waters of Loch Lomond in Scotland. In this fearful catastrophe sixty thousand human beings perished. If the disturbance takes place near the sea, great sea waves are formed, which cause fearful destruction to life and property. This happened in the case of the Lisbon earthquake; and in the year 1868, when Ecuador and Peru were visited by a fearful earthquake, a great sea wave swept over the port of Arica, and in a few minutes every vessel in the harbour was either driven ashore or wrecked, and a man-of-war was swept inland for a quarter of a mile.