EARTH TREMORS. 309 



causes. Tlius far they seem to have been recorded wher- 

 ever instruments suitable for their detection have been 

 erected, and it is not improbable that they are common 

 to the surface of the whole globe. 



Some of the more definite observations which have 

 been made upon earth tremors were those made in con- 

 nection with experiments on the deviation of the vertical 

 due to the attractive influence of the moon and sun. 



Professor Zollner, who invented the horizontal pendu- 

 lum which he used in the attempt to measure the change 

 in level due to lanar and solar attraction, found his instru- 

 ments so sensitive that the readings were always changing. 



The most interesting observations which were made 

 upon small disturbances of the soil were those of M. 

 d'Abbadie, who carried on his experiments at Abbadia, in 

 Subernoa, near Hendaye, 400 metres distant from the 

 Atlantic, and 62 metres above sea level. The soil was a 

 loamy rock. Here M. d'Abbadie constructed a concrete 

 cone 8 metres in height, which was pierced down the 

 centre by a vertical hole or well, which was continued two 

 metres below the cone into the solid rock. At the 

 bottom of this hole or well a pool of mercury was formed 

 which reflected the image of cross wires placed at the 

 top of the hole. These cross wires and their reflection 

 were observed by means of a microscope. The observa- 

 tions consisted in noting the displacement and azimuth 

 of the reflected image relatively to the real image of the 

 wires. After allowing this structure five years to settle, 

 M. d'Abbadie commenced his observations. To find the 

 surface of the mercury tranquil was a rare occurrence. 

 Sometimes the mercury appeared to be in violent motion, 

 although both the air and neighbouring sea were perfectly 

 calm- At times the reflected image would disappear as 

 if the mercury had been disturbed by a microscopic 

 earthquake. 



