1864. | Mazer on Earthquakes. 63 
when struck sharply a blow which bends them suddenly, but they 
almost perfectly resume their forms after being released from an inflect- 
ing force. 
On the other hand, India-rubber, Animal Jelly, and Whalebone, 
possess a wide range, both of flexibility and of elasticity. They recover 
their forms after great distortion, but not so perfectly as more rigid 
bodies. The elastic limit—that is, the extent to which their particles 
may be relatively displaced without fracture or other permanent alter- 
ation, is much greater in these latter, than in the former class of 
bodies. 
But we find also bodies which, like dough, or temperered potter’s 
clay, are extremely flexible, and exhibit hardly any tendency to resume 
their forms when these have been forcibly altered. 
All these are solids, 7. e. more or less rigid bodies, but liquids and 
gases are also elastic ; liquids do but very slightly—gases not at all— 
resist change of figure, but they powerfully resist change of volume ; 
and when this is altered by compression, it is restored by elasticity. 
Thus a cannon-shot that strikes the surface of the sea rises and rico- 
chets in virtue of its own elasticity and that of the water, from which 
it rebounds much farther than from a bed of solid clay or of sand ; 
but the range of the elasticity in volume, of liquids, is extremely small 
—so little, that if the weight of our atmosphere pressing upon the 
ocean were doubled, it would only squeeze about every million and 
forty-five cubic yards of water intoa million. Gases, on the contrary, 
as we all know, are largely compressible, and perfectly restore them- 
selves to their original volume; of this the air-gun affords an instance 
familiar to everyone. 
Solid bodies may be deformed by flecure, as when a carriage-spring 
is bent ; by extension, as when we pull a cord or wire endwise ; or by 
compression, as When a load is laid on the summit of a column ; or any 
combination of these may occur by the application of partial forces to 
their forms. But further, solids may be either homogeneous or 
heterogeneous, made up of different particles, or of particles having 
different elasticities in different directions. Thus, certain crystals 
have different elastic co-efficients in three different axes; and 
pseudo-crystalline bodies, such as the laminated slate of North 
Waies, or closely stratified rocks, have very different degrees of 
elasticity parallel to and transverse to the lamina, or to the strata, 
respectively. 
It is in virtue of this restorative force of elasticity, that when- 
ever a blow or pressure of any sort is suddenly applied, or a previously 
applied, steady or slowly variable force, is suddenly increased upon 
or relaxed from, any material substance, then a pulse or wave of 
force, originated by such an impulse, is transmitted through the ma- 
terial acted on, in all directions from the origin or centre of impulse, 
or in such directions as the limits of the material permit. The 
transfer through the material, or the transit of such an elastic wave, 
is merely the continuous forward movement of the original change in 
the relative positions of the particles of part of the elastic mass pro- 
duced by the extraneous force or blow—a relative displacement and 
