Page:Popular Science Monthly Volume 31.djvu/382

368 the action of those forces which produce tides in the ocean, and therefore at the same time cause a state of stress in the solid earth.

Then again it is found that earthquakes are indubitably more apt to occur when there is a rapid variation of the pressure of the air, indicated by a rise or fall of the barometer, than in times of barometric quiescence. It is certain that earthquakes in both hemispheres are more frequent in the winter than in the summer; this is probably connected with the greater frequency of sudden rises and falls of the barometer at that season. It may, however, be urged against this view that volcanic eruptions are somewhat more frequent in the summer. But whatever be the action of these external processes with regard to earthquakes, it is certain that the connection between the two is merely that of the trigger to the gun. The internal energy stands waiting for its opportunity, and the attraction of the moon or the variation in atmospheric pressure pulls the trigger. Thus the predictions of disaster which have frequently been made for particular dates must be regarded as futile.

It has long been known that an earthquake is preluded by slight tremors leading by a gradual crescendo to the destructive shocks. But within the last fifteen years it has been discovered that the earth's surface is being continually shaken by tremors, so minute as to remain unsuspected without the intervention of the most delicate instruments. In every country where the experiment has been tried, these tremors have been detected, and not merely at certain periods, but so incessantly that there is never a second of perfect rest. The earth may fairly be said to tremble like a jelly. The pioneer in this curious discovery was Father Bertelli. His experiments relate only to Italy, but that which has been found true also of England, France, Egypt, Japan, Brazil, and a solitary island in the South Pacific Ocean, probably holds good generally, and we may feel sure that earth-tremors or "microseisms" are not confined to countries habitually visited by the grosser sort of earthquakes.

Almost all our systematic knowledge of microseisms comes from Italy, for a co-operation has been arranged there between many observers with ingenious instruments at their disposal. Besides Bertelli, the most eminent of the observers is Cavaliere Michele de Rossi, who has established at Rome a "Geodynamical Observatory," and has initiated as an organ of publication the "Bulletino del Vulcanismo Italiano," in whose pages are to be found contributions from Malvasia, Monte, Cecchi, Palmieri, Egidi, Galli, and many others. The literature which has already accumulated on the subject is extensive, and it will only be possible generally to indicate its scope.

The Italians have, of course, occupied themselves largely with earthquakes, but in that field their results do not present a great deal that is novel. The instruments in use for the observation of microseisms are scarcely to be classed as perfect seismographs or