408 EARTHQUAKE PHENOMENA. 



ON OBSERVATION OF EARTHQUAKE PHENOMENA. 



BY E. MALLET, ESQ. 



[Extracted from the Admiralty Manual of Scientific Inquiry, 3d edition, 1859.] 



The observation of the facts of earthquakes and the establishment 

 of their theory constitute Seismology, (from <rettr[M$, an earthquake, a 

 movement like the shaking of a sieve,) which has only become an 

 exact science within the last twelve years. Its immediate and most 

 important applications are to the discovery of the nature of the deep 

 interior of our planet, and of the reactions of the interior upon the 

 exterior, visible in volcanic action at the surface. 



Whenever a blow or pressure of any sort is suddenly applied, or 

 the passive force of a previously steady or slowly variable pressure 

 is suddenly either increased or diminished upon material substances, 

 all of which, whether solid, liquid, or gaseous, are more or less elastic, 

 then a, pulse or ivave of force, originated by such an impulse, is trans- 

 ferred, through the materials acted on, in all directions, from the 

 origin or centre of impulse, or in such directions as the limits of the 

 materials permit. The transfer of such an elastic icave is merely the 

 continuous forward movement of a change in the relative positions, a 

 relative displacement and replacement of the integrant molecules or 

 particles of a determinate volume, affecting in succession the whole 

 mass of material. 



Ordinary sounds are waves of this sort in air. The shaking of the 

 ground felt at the passage of a neighboring railway train is an instance 

 of such waves in solid ground or rock. A sound heard by a person 

 under water, or the shock felt in a boat lying near a blast exploded 

 under water, are examples of an elastic wave in a liquid. 



The velocity with which such a wave traverses varies in different 

 materials, and depends principally in any given one upon the degree 

 of elasticity and upon the density. This transit period is constant for 

 the same homogeneous material, and is irrespective of the amount or 

 kind of original impulse. For example : in air its velocity is about 

 1,140, in water about 4,700, and in iron probably, about 11,100 feet 

 per second, all in round numbers. In crystallized or pseudo-crystal- 

 line bodies, such as laminated slate or other rocks, the transit period 

 may vary in three different directions. A very great retardation of 

 this period is produced in solids whose mass is shattered or broken, 

 even when the fissures appear perfectly close. Thus, if one stand 

 upon a line of railway near the rail, and a heavy blow be delivered at 



