308 EARTHQUAKES. 



neighbouring railway. To escape the enemy he in- 

 trenched his instruments by placing them in pits. With 

 pits 3J feet deep he found himself sufficiently protected. 

 The distance from the line was about 400 yards, and the 

 soil through which the disturbances were propagated was 

 a coarse pebbly gravel.^ 



Before the United States Naval Observatory was 

 established at Washington, Professor H. M. Paul was 

 deputed to make a tremor survey to discover stable 

 ground. The results of these experiments were exceed- 

 ingly interesting. By watching the reflected image of a 

 star in a dish of mercury a passing train would be noticed 

 at the distance of a mile. Its approach could be detected 

 by the trembling of the image before its coming could 

 be heard. At one point of observation the disturbance 

 appeared to be cut off by a ravine. The strata was gravel 

 and clay.^ 



These few examples of artificially produced tremors, 

 to which many more might be added, have been given 

 because they teach us something respecting their nature. 

 Hitherto earth tremors have only been regarded as 

 intruders, which it was necessary to escape from or des- 

 troy. From what has been said they appear to be a 

 superficial disturbance which is propagated to an enor- 

 mous distance. This distance appears to depend upon 

 the propagating medium, upon the intensity of the 

 initial disturbance, and upon its duration. In the obser- 

 vation of these artificial disturbances, which are accessible 

 to every one, and which hitherto have been so neglected, 

 we have undoubtedly a fruitful source of study. 



Natural tremors. — Next let us turn to those micro- 

 scopical disturbances of our soil which are due to natural 



^ Palmer, Trans. Seis. Soc. of Jajyan, vol. iii. p. 148. 

 2 Paul, Tratis. Seu. Sog. of Japa7i, vol. ii. p. 41. 



