Page:Popular Science Monthly Volume 31.djvu/375

Rh is written is kept moving uniformly by clock-work, we obtain also a register in time as well as space.

But in an earthquake the surface of the earth undergoes also a vertical movement which has to be recorded. The principle by which an instrument may be constructed to attain this end is as follows: If a weight hangs by a long, elastic cord, so that when set dancing up and down it oscillates very slowly, then, when a sudden jerk is given to the point of support, the weight will for the moment stand almost stationary, and a pencil attached to it may write its record on a surface fastened to the part jerked. This idea has been utilized in the construction of a vertical seismograph, but various important modifications have been introduced for the purpose of annulling the spontaneous dance of the weight after the shock has occurred.

It will undoubtedly serve to give an impulse to this science that henceforth the intending observer need not waste time in devising and constructing instruments, but can purchase the complete equipment of a seismological observatory, as recommended by Ewing, and may begin work at once.

Many other instruments besides these have been used for the observation of earthquakes, and among the best are those of Bertelli, Rossi, and Palmieri. An instrument which tells only that there has been a shock, without giving a record of the nature of the movement, is called a seismoscope. Some of the Italian instruments are seismoscopes, which, however, give an approximate idea of the severity and direction of the vibration, and others claim to be accurate seismographs or seismometers. But I do not think that any of them can compete with the instruments described in outline above.

And what do recording instruments tell us of the actual occurrences during an earthquake?

"They show," writes Ewing, "that, as observed at a station on the surface of the earth, an earthquake consists of a very large number of successive vibrations—in some cases as many as three hundred have been distinctly registered. They are irregular both in period and amplitude, and the amplitude does not exceed a few millimetres "(a millimetre is one twenty-fifth of an inch)," even when the earthquake is of sufficient severity to throw down chimneys and crack walls, while in many instances the greatest motion is no more than the fraction of a millimetre. The periods of the principal motions are usually from half a second to a second, but ... the early part of the disturbance often contains vibrations of much greater frequency. The earthquake generally begins and always ends very gradually, and it is a