"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. [Illustration: 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. [Illustration: 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.