306 EARTHQUAKES. 



CHAPTER XIX, 



EARTH TREMORS. 



Artificially produced tremors— Observations of Kater, Denman, Airy, 

 Palmer, Paul — Natural tremors — Observations of Zollner, M. 

 d'Abbadie, G. H. and H. Darwin — Experiments in Japan — With 

 seismoscopes, microphones, pendulums — Work in Italy — Bertelli, 

 Count Malvasia, M. S. di Kossi — Instruments employed in Italy — 

 Tromometers, microseismographs, microphones — Eesults obtained in 

 Italy — in Japan — Cause of microseismic motion. 



During the past few years considerable attention has 

 been drawn towards the study of small vibratory motions 

 of the ground, which to the unaided senses are usually 

 passed by without recognition. These motions are called 

 earth tremors. Their discovery appears to have been 

 due to accident, and not to the results of inductive 

 reasoning. No sooner had philosophers contrived astro- 

 nomical and other instruments for the purpose of making 

 refined measurements and observations than they at once 

 discovered that they had an enemy to contend against in 

 the form of microscopic earthquakes. 



Artificially produced tremors. — Artificial disturb- 

 ances of this description exist in all our towns, and near 

 a railway line they are perceptible with every passing 

 train. Those who have used microscopes of high power 

 will readily appreciate how small a disturbance of the 

 ground is visible in the apparent movement of the object 

 under examination. 



