39° 
A Journal  of  a Voyage  to  The  Eaft  Indies,  in  the  Ship  Grenville,  Capf.  burnet  Abercrombie,  1775,  kept  by  Alexander  dalrymple. 
2d,  The  latitude  by  obfervation ; and  where  the  latitude  could  not  be  had  by  obfer- 
vation,  it  is  deduced  by  account  from  the  lajl  obfervation , in  which  cafe  it  is 
included  within  [ ]. 
9th,  The  correB  longitude  from  Greenwich  deduced  from  the  time-keeper  corrected 
by  the  fight  of  lands,  whereof  the  longitudes  are  known,  and  by  obfervations  of  the  Moon, 
taking  a mean  of  the  feveral  obfervations  of  the  Moon  made  within  a fhort  period  of  each 
other.  The  error  of  the  time-keeper , between  the  longitude  correBed  by  fight  of  land  or 
obfervations  of  the  Moon,  is  fuppofed  to  have  arifen  by  the  time-keeper  having  altered  its 
rate  of  going  uniformly  between  thefe  obfervations ; and  the  intermediate  longitudes  are 
determined  by  the  time-keeper  on  this  fuppofition.  .Where  no  obfervations  of  the  time 
were  made,  it  is  deduced  by  the  account  from  the  lajl  obfervation  of  the  time,  and 
is  then  included  within  [ ]. 
1.0th,  The  magnet ical  obfervations  of  the  variation  and  dip , In  feven  columns. 
1 ft,  1 The  varia-r  azimuth,  j * before , denoting  the  obfervation  to  have  been  in 
2d,  J tion  by  t amplitude,  j the  morning;  * after,  denoting  the  obfervation  to 
have  been  in  the  evening.  The  variation  was  obferved  by  the  officers  with  the 
compajfes  belonging  to  the  fhip. 
3d,  The  dip  with  th  a face  of  the  injlrument  to  the  Eaft. 
4th,  Ditto,  ditto,  ditto,  Weft. 
5th,  The  mean  dip  of  the  foregoing  obfervations. 
6 th,  The  mean  corrected,  or  what  is  fuppofed  to  be  the  true  dip. 
7 th,  The  circumjlances  under  which  the  obfervations  of  the  dip  were  made. 
1 2th,  The  miles  run  by  log. 
The  dip  was  obferved  with  a dipping-needle  belonging  to  the  hon.  Mr.  cavendish, 
made  by  sisson.  * 
The  following  remarks  on  the  dipping-needle  and  obfervations  are  by  Mr.  cavendish. 
The  ends  of  the  axis  of  the  dipping-needle  are  made  conical,  and  turn  in  conical  holes 
of  bell-metal,  in  the  manner  of  Mr.  lorimer’s  needle,  defcribed  in  Phil.  Tranft  vol.  lxv. 
P-79-  The  dip  was  conftantly  obferved  both  with  the  face  of  the  inftrument  to  the  Eaft 
and  to  the  Weft,  and  the  poles  were  changed  twice  during  the  voyage,  in  order  to  fee 
whether  the  needle  continued  wrell  balanced.  The  ufe  of  this  method  of  obferving  is 
explained  in  Phil.  Tranf.  vol.  lxvi.  p.  396. 
The  mean  dip  corrected  is  what  is  fuppofed  to  be  the  true  dip. 
4 3)  correction  is  as  follows. 
The  foundation  of  this 
By  the  obfervations  on  July  1 2th,  when  the  poles  were  changed,  it  appears,  that  the 
marked  end  of  the  needle  was  too  heavy,  fo  as  to  make  that  end  point  f|ths  of  a degree 
too  low  at  that  place;  and  therefore,  if  we  fuppofe  that  the  force  of  magnetifm  is  equally 
ftrong  in  all  parts  of  the  earth,  the  error  produced  thereby  in  other  places  fhould  be  to 
r|ths  of  a degree  as  the  cofine  of  the  dip  to  the  radius.  The  obfervations  alfo  made  when 
the  poles  were  changed  at  Suez,  agree  well  enough  with  this  fuppofition:  therefore,  as  in 
all  the  obfervations  the  marked  end  of  the  needle  pointed  to  the  North,  the  mean  dip  in 
all  that  part  of  the  voyage  fubfequent  to  July  1 2th  is  corrected  by  fubtradting 
„ „ i 
± 
8 
North,  and  adding  as  much  when 
the  dip  is  as  much  South. 
J 
6 30  and  5 20 ' 
( of  a degree  from  the  mean  dip, 
5a  43 
f when  the  mean  dip  is  between 
43  . 30 
J 
. 30  0 . 
But  as  before  the  needle  left  London,  changing  the  poles  was  found  to  make  very  little 
difference  in  the  dip,  the  correction  in  the  preceding  part  of  the  voyage  is  made  not  fo 
great,  videlicet * . • 
•vA\r 
•pi  rf 
,£ll  .:v\ 
of  a degree  is  fubtra&ed  from  the  mean 
dip,  when  that  dip  is  between 
ioiis-»lle  v 
aVL  erfi  01  < 
tS«V 
70° 
and  62® 
ixi  01 .1.;.  r r) 
6a 
52  ' 
1 jfcmxbdvj; 
52 
43 
43 
35 
r I 
35 
30 
3° 
25 
,i.  T!  ill,-1 
25 
0 
. .VT  :VWu 
The  dip  was  obferved  on  board  the  Grenville  at  Deptford,  after  her  return,  in  the  fame 
part  of  the  fhip  in  which  the  obfervations  were  ufually  made,  and  was  found  not  to  differ 
more  than  5'  from  that  obferved  with  the  fame  needle  in  a pretty  large  garden  in  London, 
about  five  miles  diftartt;  fo  that  the  obfervations  on  board  the  Grenville  feem  to  be  not 
much  influenced  by  the  iron-work  of  the  fhip. 
t 6 A Journal 
