VARIETIES. 
277 
instruments,  do  more  telegraphic  work  than  one  constructed  with  copper 
wire  of  another  manufactory  of  l-16th  of  an  inch  diameter  covered  with 
gutta  percha  to  a  diameter  of  a  third  of  an  inch.  What  is  the  cause  of  these 
differences  in  electrical  quality,  is  a  question  of  much  practical  importance 
and  high  scientific  interest.  The  result  of  experiment  shows  that  the  great- 
est degree  of  brittleness  producible  by  tension  does  not  alter  the  conductiv- 
ity of  the  metal  by  as  much  as  one-half  per  cent.  A  similar  experiment 
showed  no  more  sensible  effect  on  the  conductivity  of  copper  wire  to  be  pro- 
duced by  hammering  it  flat.  There  are  no  doubt  slight  effects  on  the  con- 
ductivity of  metals  produced  by  every  application,  and  by  the  altered 
condition  left  after  the  withdrawal  of  excessive  stress  ;  but  these  are  found 
to  be  in  all  cases  so  minute,  that  the  present  results  as  to  copper  wire  are 
only  what  was  to  be  expected. — Journal  Franklin  Institute,  April,  1858, 
from  London  Mechanic's  Magazine,  July,  1857. 
The  Great  Blast  at  Holyhead. — In  presence  of  upwards  of  1,000  persons, 
a  portion  of  the  Holyhead  mountain,  which  is  122  feet  in  height,  was  dis- 
placed, on  the  21st  inst.,  by  a  blasting  operation  of  unparalleled  magnitude. 
The  removal  of  a  considerable  portion  of  the  mountain  is  necessary  for  the 
formation  of  the  new  harbor  of  refuge,  which  is  now  being  constructed 
under  the  superintendence  of  Messrs.  J.  and  C.  Rigby,  who  personally 
superintended  the  operations,  assisted  by  Mr.  G.  C.  Reitheimer,  the  resident 
engineer  of  the  firm.  Two  or  three  of  these  operations  have  already  taken 
place,  and  so  eminently  successful  were  they,  as  to  induce  the  engineers  to 
attempt  another  on  a  much  larger  scale.  The  arrangements  comtemplated 
the  displacement  of  120,000  tons  of  rock,  by  the  application  of  18,000  lbs. 
of  gunpowder.  At  the  last  moment,  the  engineers  determined  still  further 
to  extend  the  operation,  and  for  this  purpose  two  additional  chambers  of 
mines  were  prepared,  making  the  weight  of  gun-powder  used  21,500  lbs., 
and  the  body  of  rock  displaced  no  less  than  160,000  or  200,000  tons,  being 
far  more  than  could  have  been  calculated  on.  At  a  given  signal,  all  the 
chambers  were  simultaneously  ignited,  and  the  huge  body  of  the  rock  and 
mountain  was  upheaved,  and  fell  down  on  the  side  in  large  fragments  of 
several  tons  each.  It  will  be  removed  by  railway,  for  the  purpose  of  com- 
pleting the  breakwater,  from  which  it  is  about  two  miles  distant.— Jo urnal 
Franklin  Institute,  March,  1858,  from  London  Builder, 
