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EARTHQUAKE PHENOMENA. 



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through the holes in the hoop, and that they will remain 

 to mark the amount of oscillation. 



A similar apparatus, with the pendulnra-rod secured 

 horizontally (wedged into the face of a stoiit low wall, for 



>le) 



'11 

 will 



ive the vertical element of the wave. 



Two of these should be arranged, one north and south : 



the other east and west. 



It will be manifest that the observer must record 

 minutely the dimensions and other conditions of such 



apparatus, where it is not permanently kept, to enable 

 calculations of scientific value as to the wave to be made 



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from his observations of the range of either fluid or solid 



pendulums. 



A common bowl partly filled with a viscid fluid, such 



as molasses, which, on being tlirown by oscillation up the 



side of the bowl, sliall leave a trace of the outline of its 



surface, has been often proposed as a Seismometer. 



method has many objections : it can only give a 



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rude 



approximation to the direction of the horizontal element , 

 but as it is easily used, should never be neglected as a 

 check on other instruments. A common wooden tub, 

 with the sides rubbed with dry chalk and then carefully 

 half filled with water or dye stuff, would probably be the 



best modification. 



Another extemporaneous instrument for measurement 

 of vertical motion in the wave may be som.etimes useful. 

 Make a spiral spring of eighteen inches or so in length 



by twisthig an iron wire of one-eighth of an inch diameter 



round a rod of about 1| inch diameter (the staff of a 

 boarding-pike) ; suspend it by one end vertically from a 

 fixed point, and fix a weight (a twelve-pound shot will 



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