near the Grecian coast. But as data gradually accumulated he soon recognised that this occurrence could not be classed

FIG. 6. J. Schmidt's Chart of Epicentres and Outer Limits of Sensibility of the Quakes of the Eastern Mediterranean from 1846 to 1870.

with volcanic quakes, whose foci were shallow and whose effects were not widely felt, and that it must have had a much deeper origin and involved a far higher order of total energy. His data finally led him to select for the epicentre

a point far to the south-eastward (about latitude 34°, east longitude 28° 30'), as the tremors had been felt as far northwestward as Venice, and in the opposite direction far up the Nile. The whole epicentral tract, therefore, was beneath the Mediterranean, very far from shore. No unusual motion of the water was reported, but as the quake occurred in the night a slight movement might not have been observed by any one likely to make it generally known.

The quake of 1870 appears to have been on a still grander scale. It was felt along the Red Sea coast of Arabia as far as Aden. In Cairo minor injuries were reported, and also in Alexandria, where the tremors were still more forcible. The whole Syrian coast was shaken with considerable force; Crete, Greece, Smyrna, and the islands of the Ægean felt the vibrations strongly. All southern Italy and all Sicily vibrated, and the rolling of the ground was generally noted in Naples, Messina, and Palermo.

This extraordinary quake is of special interest in several respects. Its epicentre Schmidt was quite unable to locate with confidence. No sea-wave was reported from any point on the coast. On the land there was no place where the intensity was so predominant over other localities as to justify its selection. Schmidt was then impelled to assign an extraordinary depth to the centrum. If this conclusion were correct it would make this earthquake an exception to all others whose depths we have any means of judging, and none of which furnish any reason for estimating their origins at greater depth than twenty miles and perhaps not more than fifteen. The paucity of Schmidt's data is such as to forbid any conclusions on this point.

CHAPTER V

SEISMOSCOPES AND SEISMOGRAPHS

Deceptive Nature of Observations without Instruments and Necessity for the Latter Seismoscopes and their Limited Functions-Give Only an Instantaneous Phase of the Motion-Seismometers-A Steady-Point the Central Feature-Multiplying the Motion of the Tracers-Dr. Wagner's Device-Ewing's Duplex Pendulum Seismometer-Milne's Modification -Tests by Artificial Vibrations-Confusing Nature of Seismometer Traces Necessity for Seismograph to Secure a Developed Trace—The Pendulum-A Long Period Necessary-The Conical or Horizontal Pendulum-The Gray-Milne Seismograph Described in Detail-Ewing's Circular Plate Seismograph-Dr. Agamennone's Horizontal Pendulum Seismograph Described in Detail.

THE

HE first step in the experimental investigation of an earthquake is naturally to find some means of ascertaining as accurately as possible the real character of the movements which have taken place. The unaided senses are confused at the time, and the impressions they leave us, being unusual and very transitory, are only of the most general kind, and are surely misleading as to details. An instrument which can be set in motion by the quake and made to leave some automatic, intelligible record is needed. The earliest instruments designed for this purpose were of a very primitive character and were, moreover, based on a misconception of the motions of the ground during the earthquake. In most countries, especially those of northern

Europe and our own Atlantic States, such an occurrence is the event of a lifetime, or by a large proportion of mankind is not experienced at all. With most of the race knowledge of them is derived wholly from written accounts, and these, until seismology became a science, were vague and more or less imaginative. It is not surprising, therefore, that the notion prevailed that an earthquake consisted of a single "shock," or a very few shocks, instead of a sustained series of many hundreds of vibrations, as we now know. Conformably to this notion the earliest instruments were designed merely to record the fact that an earthquake had passed and the direction in which its impulse or "shock" had acted; for that the shock had some determinate direction was not doubted. Instruments of this class are termed seismoscopes. They do nothing more than announce the fact of an earthquake, and sometimes also the instantaneous direction of the force which set them in motion.

To devise something which will signal a decided tremor of the ground is the easiest kind of problem. Anything which trembles moves, and a very slight motion can be made to dislodge a delicately poised object, or to close an electric circuit so as to animate a magnet, or to start undulations in a dish containing a liquid. The number of such devices which have been experimented with or suggested is almost countless. But for the purposes and methods of modern research they are of slight value. Though of much historic interest they may engage but little attention here. Any cyclopædia, article "Earthquake," will give as much information about them as is really essential, and it is hardly worth while to repeat it.

The seismoscope, whatever be its mechanical construction, can give us only the result of an instantaneous phase of the movement of the earth, or of the support on which the instrument rests. In order to gain a more instructive idea of the movement we need an instrument which will trace the whole motion automatically from the beginning of the quake to the end, showing at every instant the direction, the amplitude, and the frequency, or “periods" of the vibrations. Such an instrument is termed a seismograph. There is an intermediate class of instruments which give continuous records of the movements of the ground and also show the directions and amplitudes, but they give them in such a complicated, tangled form that it is impossible to unravel them and show each vibration distinct from the others, as may be done with the record of the seismograph. These instruments are usually called seismometers.' Their results are very instructive and useful, and as the machinery is far simpler, less expensive, and more easily maintained than that of the seismograph, they are often very desirable. Their distinctive features will appear more clearly farther on.

The fundamental problem in all seismometers, including the seismograph, is to devise some object or mass which will remain at rest while everything around it and the very support which upholds it is in a constant state of vibratory motion. Such a mass is called a steady-point. To attain it is far from easy. But if it can be attained then we can conceive of a plate attached to it with a prepared surface capable

The name seismometer seems to be generic rather than specific, the seismograph being a species of seismometer, and the generic name being applied specifically also to the less complex instruments.