78 EARTHQUAKES. 



of an earthquake, we usually employ terms like 'weak,' 

 * strong,' 'violent,' &c. Although these expressions, ac- 

 companied by illustration of the effects which an earth- 

 quake has produced, convey a general idea of the strength 

 of a shock as felt at some particular locality, our ideas are 

 nevertheless wanting in definiteness ; and if we endeavour 

 to compare one shock with another, as a whole, our want 

 of exactness is augmented. We have seen that Palmieri's 

 seismograph indicates intensity by a certain number of 

 degrees, which, to a certain extent, is a measure of the 

 violence of the motion as indicated at a particular locality. 

 The degrees, as before stated, refer to the height to which, 

 in consequence of the shaking, a certain quantity of 

 mercury was washed in a tube, which is a function of the 

 depth of mercury in the tube, and also of the duration of 

 the disturbance. 



From this it seems possible that a very slow motion 

 of small amplitude, continuing over a sufficient period of 

 time, might, if it agreed with the period of the mercury, 

 indicate an earthquake of many degrees of intensity, 

 whilst residents in the neighbourhood might not have 

 noticed the disturbance ; and, on the other hand, a short 

 but intense shock creating considerable destruction might 

 have been recorded as of only a few degrees of intensity. 



Although objections like these might be raised to such 

 a method of recording intensity, in practice it would 

 appear that such results are not pronounced, and the 

 indications of the instrument usually give us approximate 

 indications of relative intensity. 



In writing about the Neapolitan earthquake of 1857, 

 Mallet says that 'area alone affords no test of seismic 

 energy.' 



The area over which a shock is felt will depend not 

 only upon the initial force of the disturbance, but also 



