APPENDIX. 



Simple instbtjctions foe earthquake obsee vat ions. 



The foregoing pages, which contain many notices and extracts which are fair samples 

 of ordinary unscientific descriptions of an earthquake, show well how little is known 

 by most of the true nature of an earthquake, and consequently what very imperfect 

 observations it is possible to collect in a country so well suited for systematic seismo- 

 logical enquiry as the Assam province, and, to a less degree, nearly the whole of 

 India ; and yet there are few who could not make observations which, when collated 

 with those made by other observers in different positions, would not be of considerable 

 value. 



As it is impossible to make observations of an earthquake, which will be of any 

 use, without some previous knowledge of what an earthquake really is ; and as, even 

 up to the present day, the common manuals of geology contain erroneous or misleading 

 notices of this branch of terrestrial physics, — it will be advisable to devote some space 

 to the description, in as few words as possible, of what an earthquake really is ; and on 

 this point the vaguest ideas and wildest impressions are prevalent — not unfrequently 

 it is spoken of as a heaving of the solid earth, caused by the pent-up forces beneath, 

 which find an outlet in the fissures and rendings of the earth which they produce. 

 Now this, which may be looked upon as a fair and impartial description of the ordinary 

 idea of an earthquake, could not well be more at variance with the facts than it is. 



Suppose a long beam of wood to be lying on the ground — against one end of this 

 hangs a small weight suspended by a string, the opposite end of the beam being struck 

 by a hammer this weight is driven away from the wood without the beam being moved 

 the fraction of an inch; what has happened is that a wave of elastic compression has 

 been sent through the beam : when the end was struck by the hammer the particles 

 were driven before it, but by their elasticity recovered their original position ; mean- 

 while they had pushed forward the particles immediately in front of them, which, in 

 their turn, having communicated the motion to those in front of them, returned to their 

 original position, and so the impulse was transmitted through the beam, and on reach- 

 ing the opposite extremity the outermost particles were pushed forward, and, in doing 

 so, communicated their motion to the small weight hanging against the end of the 

 beam, which was in consequence thrown off: it is just such a wave of elastic compres- 

 sion which passes through the earth's crust when what is commonly called an earth- 

 quake takes place. 



We must now explain how it is that we can, from observations at the surface, deduce 

 the position and depth below the surface of the focus from which the shock originated, 



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