3 68 THE POPULAR SCIENCE MONTHLY. 



the action of those forces which produce tides in the ocean, and there- 

 fore 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 baro- 

 metric 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 sum- 

 mer. 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 pre- 

 dictions 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-tre- 

 mors 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 ob- 

 servers 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 litera- 

 ture 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 micro- 

 seisms are scarcely to be classed as perfect seismographs or seismom- 



