There are three classes of instruments for the automatic recording of earth tremors, each with its own particular function.  First is the seismoscope, which will merely detect and record the fact that there has been such a tremor.  Some of these are so equipped as to indicate the time of the disturbance.

Second, is the seismometer, the function of which is to measure the maximum force of the shock, either with or without an indication of its direction.  The third instrument is the seismograph, which is so arranged that it will accurately record the number, succession, direction, amplitude and period of successive oscillations.  This last instrument is by far the most delicate of the three.

In the construction of this earthquake recording machine the maker must so suspend a heavy body that when its normal position is disturbed in the most infinitesimal degree no reactionary force will be developed tending to restore it to its original position.  The inventor has never been found who could accomplish this suspension of a body to perfection.  The seismograph of to-day, however, has reached a stage of perfection where close approximations are obtained in the records made.

CHAPTER XV.

Vesuvius Devastates the Region of Naples.

We have in other chapters described the terrible work of Mount Vesuvius in the past, from the far-off era of the destruction of Pompeii down to the end of the last century.  There comes before us now another frightful eruption, one of the greatest in its history, that of 1906.  For thirty years before this outbreak the mighty volcano had been comparatively quiet, rarely ceasing, indeed, to smoke and fume, but giving little indication of the vast forces buried in its heart.  It showed some sympathy with Mont Pelee in 1902, and continued restless after that time, but it was not until about the middle of February, 1906, that it became threatening, lava beginning to overflow from the crater and make its lurid way down the mountain’s side.

It was in the middle of the first week of April that these indications rose to the danger point, the flow of lava suddenly swelling from a rivulet to a river, pouring in a gleaming flood over the crater’s rim, and meeting the other streams that came streaming down the volcano’s rugged flank.  While this went on the mountain remained comparatively quiet, there being no explosions, though a huge cloud of volcanic ash and cinders rose high in the air until it hung over the crater in the shape of an enormous pine tree, while from it a shower of dust and sand, soon to become terrible, began to descend upon the surrounding fields and towns.

Dangerous as is Vesuvius at any time, the people of the vicinity dare its perils for the allurement of its fertile soil.  A ring of populous villages encircles it, flourishing vineyards and olive groves extend on all sides, and the hand of industry does not hesitate to attack its threatening flanks.  The intervals between its death-dealing throes are so long that the peasants are always ready to dare destruction for the hope of winning the means of life from its soil.