CHAPTER XXXI

INSTRUMENTS FOR RECORDING AND MEASURING EARTHQUAKE SHOCKS

To attempt by the unaided senses a determination of the direction in which earthquake shocks reach any certain spot, the velocity with which they are travelling, their degree of intensity, their general character, whether horizontal or vertical, or any peculiarities which might show them to be exceptional would be futile for more reasons than one. Even a skilled scientific observer, familiar with what has already been discovered and eager to discover more, might in the excitement of an earthquake become so excited himself as to make him unable to take reliable observations.

But human ingenuity has succeeded in devising delicate instruments capable of recording not only the exact time of the arrival of an earthquake shock, but also of measuring the different parts of what may seem to be a single shock, the direction in which the shocks reach the place, as well as the variations of intensity in all the shocks.

Crude instruments to do some of these things have been in use from very early times. According to Mallet among the more important of these early instruments was the following: the instrument of Cacciatore of Palmero. This consisted of a circular wooden dish, about ten inches in diameter, placed horizontally, and filled with mercury to the brim of eight notches at equal distances apart. Beneath each notch was placed a small cup. On the passage of the earthquake waves the vessel, being tilted in a direction dependent on the direction in which the waves were travelling, would cause some of the mercury to spill over into one or more of the cups, thus indicating by its amount the intensity of the wave, and by the particular cup or cups that were filled, the direction in which the waves reached the place.

Somewhat similar contrivances were of a vessel partly filled with molasses, or other sticky liquid; or a cylindrical tub, the sides of which were chalked or whitewashed and filled with some colored liquid. In either of these cases, on the passage of the earthquake waves, the vessels were tilted and showed by the height of the marks the intensity of the waves, and by the position of the marks the direction in which the waves first reached the instrument.

These instruments, though satisfactory for the study of earthquake shocks a long time ago, when an earthquake was regarded as practically consisting of but a single shock, or, at the most, of a very few shocks, would be worthless for the study of earthquakes now, for it is finally known that an earthquake consists of a series of many hundreds of vibrations, differing greatly in their rapidity and intensity, and following one another in a definite order.

The old forms of earthquake instruments were known as _seismoscopes_. The word seismoscope is a compound word from Greek consisting of the two words, seism and scope. It means literally any instrument capable of seeing, or calling attention to, a seism, or _earth-shake_. In other words, a seismoscope is any instrument capable of calling attention only to an earth-shake.

Of course, neither of the rude seismoscopes just mentioned would be able to give any valuable indications of the successive shakings to which the vessel containing the viscid liquid had been subjected, since the liquid would simply be splashed a number of times over the same parts of the vessel. In order to get a record of the successive shocks another form of apparatus must be employed, a form known as a _seismograph_.

Concerning the complex character of the apparently single earthquake shock, Professor Milne makes this highly interesting and picturesque statement:

"An earthquake disturbance at a station far removed from its origin shows that the main movement has two attendants, one which precedes and the other which follows. The first of these by its characteristics indicates what is to follow, whilst the latter, in a very much more pronounced manner, will often repeat at definite intervals, but with decreasing intensity, the prominent features of what has passed. Inasmuch as these latter rhythmical, but decreasing, impulses of the dying earthquake are more likely to result from reflection than from interference, I have provisionally called them Echoes."

There are many different forms of instruments known as seismographs that are capable of recording all of these vibrations, but there is this objection to their use: that the records appear in so tangled a form that it is practically impossible to decipher or untangle them. This fact can be grasped by examining Fig. 51, which represents a record of this kind.

It is necessary, therefore, to employ a modified form of instrument called a _seismometer_, able not only to record all the different vibrations, but to record them in such a manner that they can be easily recognized. Fig. 52, for example, shows results obtained by the use of a seismometer, in which the different vibrations are separated, and so recorded on a sheet of paper, as to be readily understood. Such a record is called a _seismogram_, and represents a _long distance seismogram_. Here the large arrow indicates the beginning of the record. And herein, as can be clearly seen, what would appear to an observer without an instrument only a single shock, lasting but the fraction of a minute, in reality consists of the _preliminary shake_ as represented in ab and bc, the _principal shake_, as represented at c, d1, d2, and d3, and the _final portions of the shake_ or the "echoes" of Professor Milne, as represented from d3 to e.

Except in a very general way there is for present purposes no need of explaining the construction and operation of the seismometer and seismograph. Suffice it to say, there are many forms of these instruments, any of which are capable of recording the details of a passing shock. The most important thing in either a seismograph or a seismometer is to obtain what is known as a _steady point_; that is, a point consisting of some object or mass that will remain practically at rest, while everything around it, even the support which holds it, is affected by the earthquake.

It is, of course, not very easy to obtain a steady point, but it can be done; and it will be at once comprehended that if a plate or piece of paper were attached to such a steady point or mass, and a pencil or tracer had one of its ends resting on the plate, and its other end attached to the support that vibrates with the earth, a tracing or record would be drawn on the plate from which the character of the motion of the end of the tracer, and, therefore, of the earth, would be marked on the plate.

Various devices have been employed for the steady points. The most successful consists of a heavy mass of lead.

Fig. 53 represents a form of instrument invented by Professor Vicentini of Italy. Here the steady point consists of a heavy leaden bob, of 200, 400, or even 500 kilograms, suspended by three metallic rods united above by a brass cap, hung on a steel wire to a bracket fixed on the wall. This wire may have a length as great as fifty feet.

Fig. 54 represents the recording instrument. Here a tracer is provided that is capable of multiplying the motion fifty times, or even eighty times. A record is traced on a sheet of paper passing over a roller which imparts a rapid motion to a sheet so as to make sure that the different parts of the shock or vibration will be recorded on separate portions of the paper.