256 TEAR-BOOK OF FACTS. 



We have not space to quote the details. The principle upon which 

 the instrument is constructed is very simple. It consists of an 

 arrangement of a large pendulum, an inclined bar, and a number of 

 small pendulums, the latter terminating in sharp needles, which 

 touch ashes placed beneath them, and upon which are left traces of 

 the oscillations. Now, the instrument being capable of marking, 

 1st, the vertical altitude of the wave by the spiral pendulum ; 2nd. 

 the horizontal undulation by the great pendulum ; 3rd, the time of 

 the wave as marked by one or other of the small pendulums, we 

 have all the elements necessary for calculating the intensity of the 

 shock — lastly, it is clear that with these three elements we can 

 make all possible theoretical inferences, and assign to each of the 

 three its appropriate value in referring to the effects of an earth- 

 quake, whether on buildings, on plains, or on the sea, &c, in all of 

 which one or other of the three mentioned powers will have a greater 

 or less influence : it has been proved, for example, that with an 

 equal degree of intensity, the vertical shock will do more damage 

 than the horizontal. Thus we can note with these instruments — 



1st. The moment at which the earthquake incurs. 



:2nd. The direction of the primary shock i>r earth-wave, 



3rd. The general horizontal direction of the waves, their amplitude or length. 



4th. The height of the vertical wave of shock, however complex the vertical 

 and horizontal waves acting together may be. 



">th. The resultant of both these elements, or the mixed shock itself. 



8th. The inclination to the horizon of the mixed shock. 



7th. The velocity and time of the wave. 



Bth. The tots] intensity of the wave, introducing into it the element of I 

 furnished by the pendulums. , 



If, then, as frequently happens, we also know the total dui 

 of the earthquake, we may approximately infer its total intensity. 



BABTHQUAKR8 IX HAPLBR 

 Mr. Rohekt MALLET, who, in 1859, undertook a journey to 

 Naples, to study on the spot the results of the terribly disastrous 

 Earthquake which occurred in that covntry, has prepared a volu- 

 minous dooumenl thereupon, in which the whole matter is discussed 

 in its several hearings, tie has reported from time to time to the 

 British As Bubjeot of Earthquakes generally, and 



from his last observations, can tell what was the primary direction 

 of the shook, its subsequent variations, direction ot emergen 

 rate of movement. A competent observer, by going over the 

 ground, would now be I off all these phenomena as from a 



book. Of all the directions in whieb i travel, the hori- 



zontal is the mo ' .A shook or wave hai in [ rist d to tin- 



arface, becomes more and i re horizontal as it travels, and would 

 throw down everything id il I it not become weaker with 



. furlong of distance. The shock thai overthrew towns and 

 res iii Naples had s movem 

 than this suffices for i irding to Mr. .Mallet, a 



horizontal w lling at foui eond onlj would I ave 



London a heap of bricks. — ijhumbcrts ■' 



