490      M.  F.  Zollner  on  the  Origin  of  the  Earth's  Magnetism, 
may  be  different  according  to  its  strength  and  direction,  and  ac- 
cording to  the  condition  of  the  inner  surface  of  the  solid  crust. 
All  phenomena  appear  here  which  may  be  produced  by  the  pro- 
pagation of  waves  with  unequal  velocities  &c. 
"  The  author  concludes  with  remarks  and  calculations  on  the 
frequency  of  earthquakes  during  the  latter  half  of  the  eighteenth 
century,  in  relation  to  the  position  of  the  moon,  and  on  the  fre- 
quency of  the  phenomenon  with  regard  to  the  passage  of  the 
moon  through  the  meridian.  The  greater  frequency  at  the  time 
of  the  syzygies  in  contradistinction  to  the  quadratures,  and  in  the 
perigee  in  contradistinction  to  the  apogee,  is  confirmed." 
The  indirect  influence  of  the  sun  would  be  shown  by  the  in- 
fluence of  the  sun-spots  on  the  frequency  of  the  earthquakes.  I 
do  not  know  of  any  fact  to  that  effect. 
I  take  the  liberty  to  communicate  a  passage  from  a  memoir 
by  Mallet*  (in  the  words  of  the  reporter  of  the  Berliner  Berichten, 
1863,  p.  926),  respecting  the  undulatory  character  of  the  earth- 
quake observed  in  England  during  the  night  of  October  5-6, 
1863.     _ 
"  With  respect  to  the  explanation  of  the  conception,  Mallet 
expresses  himself  decidedly  against  the  opinion  that  the  earth- 
quake is  a  means  to  produce  lasting  geological  upheavings.  He 
thinks  it  is  rather  the  passage  of  a  wave,  or  of  several  waves  of 
elastic  compression,  in  any  direction,  from  the  vertical  to  the  ho- 
rizontal, in  every  azimuth,  through  the  mass  and  surface  of  the 
earth,  commencing  from  one  or  more  centres  of  percussion. 
Tones  and  surging  motions  can  be  traced,  which  depend  on  the 
given  percussion  and  on  the  distribution  of  land  and  water. 
"  Mallet  then  gives  a  short  abstract  of  the  history  of  earth- 
quakes. The  right  view  of  them  has  only  been  obtained  since 
the  brothers  Weber,  and  Scott  Russell,  developed  the  laws  of  cer- 
tain undulatory  motions,  and,  chiefly,  the  latter  the  laws  of  the 
motion  of  translation.  He  then  considers  the  phenomena  of 
earthquakes  up  to  a  certain  point." 
It  is  to  be  seen  from  these  data  that  the  movements  of  the 
ground  may  be  perhaps  continuous,  but  then  only  so  weak  that 
they  cannot  be  observed  without  more  delicate  aids.  At  any 
rate  we  may  assert  that  the  frequency  of  positively  ascertained 
movements  of  the  ground  would  be  considerably  increased  if  we 
possessed  delicate  seismometrical  instruments. 
24. 
Two  years  ago  I  had  shown  and  explained  to  the  Royal  Society 
*  "  The  late  Earthquake  and  Earthquakes  in  general,"  Quarterly  Journal 
of  Science,  vol.  i.  pp.  53-69  (1863). 
