252 Notes on the Recent Earthquakes [No. 123. 



of shock longer than it really is, and in most instances, considerable 

 deductions might with safety be made from recorded observations on 

 this point, to bring them near the truth. The duration of intervals 

 between shocks should also be carefully noted. 



3. Nature of the Shocks. — This is very frequently twofold : one kind 

 throws the crust of the earth into a tremulous state. This was the 

 nature of the shock of the 19th February. The second kind is of the 

 nature of a concussion or blow, and does not always occur. Sometimes 

 both of these are conjoined in one and the same shock, and the latter 

 is felt generally in the middle of the former. 



4. Nature of the motion on the Earth's surface. — Three different 

 varieties of this have been observed. First, a horizontal motion by 

 which bodies are, as it were, pushed horizontally forward. Second, a 

 vertical motion by which they are lifted up and dashed down again. The 

 conjunction of these two kinds of motion produces the third, which 

 is of an undulatory character, partaking both of the horizontal and 

 vertical movements. This kind is the most frequent of all, and produces 

 those sensations of nausea, so commonly alluded to. 



5. Rents in the ground and suosidencies are very common accompani- 

 ments of Earthquakes, and their appearance ought to be represented 

 on paper, and their dimensions carefully measured. These are often 

 accompanied by loud noises of various kinds. 



6. Meteorological phenomena are highly important, and some curious 

 and interesting relations have been observed, between these and the oc- 

 currences of Earthquakes. This is especially true as regards the state of 

 the barometer and thermometer, and the electric condition of the 

 atmosphere. Such points therefore merit peculiar attention. 



7. Geological structure of affected District. — When the observer is 

 qualified to furnish information relative to this, his remarks will be addi- 

 tionally important, as it has been observed, that in localities exhibiting 

 certain geological features, Earthquakes always occur with much 

 greater frequency than in others. Wherever powerful and extensive 

 volcanic action has occurred, where faults and fissures communicating 

 with the internal seats of disturbing forces are found, there Earthquakes 

 occur with greater frequency and higher intensity, and they are fre- 

 quently observed to pursue a direction, parallel to that of the principal 

 faults or fissures. 



