Page:Popular Science Monthly Volume 69.djvu/122

118 other hand, the equally large earthquake of January 31, last, the origin of which was at sea off the west coast of Ecuador, besides recording itself on seismographs the world over was recorded on the magnetographs at Baldwin, Porto Rico and Cheltenham, but this time not at Honolulu. This seaquake was accompanied by a tidal wave twenty feet high which rushed in on the coast of Ecuador, causing great devastation; it set the Pacific Ocean in vibration, which according to the tide-gauge records of the Coast and Geodetic Survey at San Diego and Honolulu lasted for three days. The tidal wave, when it rushed in on the Hawaiian coasts, was several feet high, and the record of this quake of January 31, as recorded on the Milne seismograph at the Honolulu Magnetic Observatory, was among the largest since the installation of the instrument, September, 1903, and yet the delicately suspended magnets, as far as the magnetic records at this observatory would indicate, were not affected.

Why is it that an earthquake will at times be recorded by magnetic instruments and at other times leave no record? Or, to go back to the fundamental question, what do the magnetic instruments record—an actual mechanical effect due to the mechanical vibration of the point of support? If the observed effect is a purely mechanical one, then why is it that not every mechanical disturbance is recorded on the photographic records of the fluctuations of the magnetic needles? What is the characteristic of the mechanical vibration, the presence or absence of which in the earth movements is responsible for the presence or absence of the effect recorded by magnetic needles?

The solution of these questions may show the magnetograph to be a most useful adjunct to the present instrumental equipment for recording earth movements.

Is the possibility of any actual magnetic effect accompanying an earthquake entirely to be excluded? If so, in the case of the distant earthquakes, as seems probable, is the possibility also to be excluded for the less distant ones, or say for stations within a certain prescribed region about the origin of the quake? Are those cases where records are secured on magnetographs and not on seismographs to be attributed possibly to such a magnetic effect which has no influence on instruments responding merely to mechanical vibration? Or is it possible that the magnetograph is in certain cases a better micro-seismograph than the Milne or Bosch-Omori instruments used in this country?

We have thus some extremely interesting questions presented to us which, however they may be solved, will be a valuable contribution to our knowledge of earth movements. The possibility might also be mentioned that an approaching earthquake might through electric or magnetic effects give the first indication on magnetographs because of the much greater velocity of propagation of such effects than that