s 



;i|i|in'ii;il)l(' time. Mure (•(imnioiilv, liowi'Vi'i', tlu' >f('i)ii(larv \ilii':itioiis due to 

 (loiihlc rctlwtiiiii ill a stiatiiin will liavo too lonji; a period to give rise to a cou- 

 tiniiiins sound. 



li'tiic sides iif tile stratum are iKit ]iara]lel, the period and also the dircetiou 

 of the Secondary waves (p|iserv<'d at any station in or beyond the stratum will 

 ehaniie progressively, and a like eifeel will lie jirodueed oven by a uniformly 

 thick stratum when tile fnmt of the incident wave is not plane but curved. The 

 figures given abuve illustrate these cases. 



i? J^. Hß'ecf (if iihslarli s. Knrlliijiiiikf Shcidiiiv--! mid Diffructioil. 



A\ hen \\a\es (lavelliiig tiiniiigii an otheruise lioiiingeiieous medium impinge 

 upon a space .)i'cii])ie(l by matter whose mechanical properties (as to density, 

 compres-^ibility or rigidity) differ I'roiii the properties of the medium round it, the 

 Ibreign body moves so as to act as a new source of disturbance, and secondary 

 waves ai'c thrown oil' from it in all directions. These are of <-oiirsc generated at 

 the expense of tiie energy of the original sy.stem of waves. 



When the dimensions of the foreign body are sufficiently iiierea.<ed, reflec- 

 tion and refraction take jilace at its surface according to the princi])les already 

 stated. If we suppose it lu h,. impervious to waves of the particular class which 

 strike it, it will act as an obstacle to shield from disturbance a jiortion of the 

 meilium lying beyond it, and may therefore be said to cast an earthiiiiake shadow. 



The geometrical outlines of the eurthipiakc shadow will of course be deter- 

 mined by drawing lines parallel to the direction of (he incident waves and 

 tangent to the sides of the obstacle: but neither will this region be comjih^tely 

 slii<>l(led from disturbance nor will the surrounding portions of the medium be so 

 inteiiselv disturbed as (liev would be were the obstacle absent. In other words 



