EARTHQUAKE MOTION DEDUCED FEOM OBSERVATION. 73 



rocks, produces in them tremors which are to be observed 

 at great distances. Of this the author convinced himself 

 at the Falls of Niagara, where he observed the reflected 

 and ever-moving image of the sun in a pool of water. 

 Under favourable circumstances almost continual conden- 

 sation of steam might take place in volcanic foci, each 

 condensation giving rise to a blow sufficiently powerful 

 to produce vibrations in the surrounding ground. Those 

 who have stood near a large geyser, like the one in 

 Iceland, when it makes an ineffectual effort to erupt, 

 will recognise how powerful such a cause might be. 

 Humboldt has remarked shocks on Vesuvius and Pichincha 

 which were periodic, occurring twenty to thirty seconds 

 before each ejection of vapour and ashes. 



Earthquakes like these may be of vast extent, gradu- 

 ally spreading further and further outwards. This 

 spreading of earth vibrations may be observed at a large 

 factory containing heavy machinery or a steam hammer. 

 After the machinery comes to rest, it is probably some 

 time before the ground returns to rest. Examples of 

 disturbances of this nature are spoken of under the head 

 of Earth Tremors. 



The record of the duration of an ordinary earthquake 

 as observed at a given point is dependent upon the sensi- 

 bility of our instruments. 



Continuous motions perceptible to our senses without 

 the aid of instruments usually last from thirty seconds to 

 about two or three minutes. In Japan the shocks, as 

 timed by watches, usually last from twenty to forty seconds. 

 Occasionally a continuous shaking is felt for more than 

 one and a half minutes, and cases have been recorded where 

 the motion has continued for as much as four minutes and 

 thirty-three seconds. 



Seismometers having a multiplication of 6 to 12 



