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Sect. VIL] 



EARTHQUAKE PHE 



^NOMENA 



213 



the horizon bodies are often projected, as stones out of 

 or from the coping of walls: the size, weight, form, 

 cement, sort of stone, distance thrown, and all otlier con- 

 ditions of projection should then he carefully noticed. 



twisted on a vertical axis (such as the Calabriaa 



Bod 



;S 



Geology ') 



s^ 



upposed 

 due to a vorticose motion of the earth. This motion 

 arises fr jm the centre of gravity of the body lying to one 

 side of a vertical plane in the line of shock, which passes 

 through tliat point in the base on which the body rests, in 

 which the whole adherence, by friction or cement of the 

 body to its support, may be supposed to unite, and wlrich 



of 



The observer who 



fully masters these mechanical conditions of motion will 

 see what elements he must collect, so that the motion 

 impressed on bodies thus twisted may be used to calculate 

 the velocity, &c. of the wave. All observations of this 

 class, to be of scientific value, must comprise the ma- 

 terials, size, form, weight, sort of cement, base or founda- 

 tion of the bodies disturbed, and measurements of the 

 amount, &c. of disturbance, with any other special con- 

 ditions which occur; and these will always be very 

 numerous, and demand the utmost alertness and scrutiny 



of the observer. 



Amongst the doubtful phenomena on record of this 

 class are inversions of bodies, such as pavements turned 



upside down (see Lyell, ' Calabrian Earthquake ') 

 such cases deserve special attention. 



In traversincr an extensive city, or thickly built-over 

 country, to observe the shattered buildings— having first 

 known the general line of motion of the wave— the 



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