70 
HAYFORD. 
region at that particular time the earth is failing. As the 
sensitive seismographs now in operation show that there are 
hundreds of earthquakes each year, the sum total of such 
failure is not small. But I repeat that the observations of 
earthquakes furnish little or no evidence of the absolute 
strength of the earth ; for in or near each earthquake center, 
where the test of strength occurs, no accurate observations 
are ordinarily secured. Outside the region in which the 
test of strength takes place, seismographs secure a record of 
the elastic vibrations, and thus secure evidence as to the 
earth’s rigidity — its stiffness, hut not its strength. Tf the 
elastic vibrations were recorded by a seismograph so close to 
an earthquake center that the stresses during the vibration 
approached the breaking limit, then some information as to 
the strength might be secured; but such a fortunately located 
seismograph is so badly shaken that it furnishes no readable 
record. 
If the earth is a failing structure; if throughout consider- 
able portions of its mass the stress-differences are so great 
that the material is yielding in a non-elastic manner, or is 
about to yield in that way, if the sudden motions at earth- 
quake centers are examples of such yielding, and if such 
stress-differences are due to the weight of the continents, or 
to other causes which act for geological ages with but little 
change, then we should find that earthquakes are more fre- 
quent at times when stresses due to other than these secular 
causes are greatest, and less frequent when the temporary 
stresses are least. According to the conception I have put 
before you, the earth is like a revolver with a hair-trigger. 
Just as a slight touch on the hair-trigger will discharge the 
revolver, so very slight unusual stresses in the earth should 
tend to produce earthquakes. This is apparently the fact. 
Omori, the Japanese earthquake expert, states that, several 
such relations exist.* At times of high barometric pressure 
earthquakes of land origin are unusually numerous. Earth- 
* Publications of the Astronomical Society of the Pacific, vol. xvm, 
No. 109, August 10, 1906, pp. 235-241. 
