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.
Fig. 51. Complex Record of Seismograph
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.
Fig. 52. Long Distance Seismogram
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.