DIPPING, 
Fixed upon a aaa pears ne epee — horizontal 
by means A Pie 8 and a level o 
Fig. 2 
e, the ax's of w hich, EF, 
which 
the ftand which 
Ww ith {uch a fland the inf 
conuee 
The ‘belt method of obferving the hela is to Jal the 
needle in the — meridian by m of a on 
compafs, taking c o place the two a anen s fat. 
ficiently d:ftant ae wach other, left they 
each other’s movements. Th 
dipping-necdle makes on 
frame, which contains the dipping- eal is fornithed with 
two fight v vanes on an index, which moves horizontally on 
the top of the frame, and ee thofe — a diftant ob- 
ject, the bearing of which is exa@ly wn, may be kept 
in view, in ie to fituate ie poeneparing. in the mag- 
netic meridia 
ith fu are an ioftrument the error of obfervation, which 
fhould influence 
en oe the ee w oa - 
the hor 
witt needle downwards; and a 
‘mean of the two obfervations is taken as the true or corre 
di B-tides this, the needle may bé turned with each of 
its two flat furfeces, alternately to ‘the right and to nie left, 
o make, alt oge ether, four peat aaa a mean of 
whi ch will come much nearer to th 
ut t 
It is almoft ufelefs to remark, eas in the conftruétion of | 
bfer 
fuch inflruments, as well as in making th vaticn, no 
iron, ftecl, or other ferruginous matter, ae be fu fered to 
‘be in the frame, or about it, fince a very ey of 
it ts panei to render the oblciiation erro 
A great imperfeCtion in the dipping Saeed arifes from 
the di ferent ee with which the magnetifm ifm t 
acts upon its two extremities, in differe 
tion. This fource 
e 
t ich refts upon 
the fupport CD; and E F is the line ‘which pall 
the centre of the needle, and divides it into tw 
pafles avey through the centre of the needle, and 
divides it into equa 
perfectly bala anit in any degree of inclination. 
the needle be perfe@tly bala ghee yet itis manifeit, that in 
‘an inclined Given: as fhewn i 
which ftands abo 
perpe icant a that part of it, which, lies above 
the {upp plane, exceeds the other part juft by the 
diam co Poe ie ae Now when a needle is inclined, as 
in fg. 26, fappofe that two equal and like forces be applied 
to its extremities A and B, it is pions ee se a aps 
plied to the extremiry A, ave mcre 
rd needle than that hich is aon Ned to OB, peeaufe G A is 
e longer lever of t This inequality of effe&s pal 
aries increafe pane the as the inclination increafes, a 
is greatcft whcn the reedle ftands ina perpe endicular Gi 
tion. The application of thefe forces takes place when th 
— has rendered magnetic ; for, as the greatelt at- 
radlive and repulfive powers, between ihe magnetic poles 
of the earth are the a of the needle, a& upon the ex- 
tremitics A and e above-mentioned irregularity muft 
Increafe with the ‘selination of the needle, and with the 
dexree of its magnetic power o effeétual method of re- 
moving this caufe of error has, as yet, been devi 
f the general con- 
mmon imperfe@tions of this delicate 
magnetical inftrument, it is now necellary to defcribe the 
beft or mofl improved inflruments-of the kind, and to fub- 
join the moft approved precautions, Wis oe fhould be ob- 
ferved in the ufe and mana age ok of ame 
Rev. Mr. Mitchell, a gentleman oat ae 
iC. long after, 
milar fit vuriont for the 
Royal Soctey i which Tae. differed in fome particulars 
from the for 
wards be eafy to point out the a avticulace | in + which the 
dipping see of the Royal Society differs fro 
Fig. 24. is a delineation of one of the needles, made for the 
€ 
A is the needle, whofe length is 
e the ends of its axis, gs re 
en 
they all bear againtt flat ces agate, = nely 
polifhed. The ends of the axes, of the 
“holes made in bell metal. 
which is rt ee into d 
cite of fhewing the angle 
with the aonen: It is hardly neceflary to obferve, that the 
centre of the needle coincides with the centre 0 the di- 
which form the 2 arms of this por is cut with a fine {crew, 
to receive a {mall weight, or button, which may be i 
nearer to, or farther from, the axis of motion 3; whence 
oe ere be adjufted both ways with great nicety, “sigs Tes 
erfing the polee, and changing its fide. G, G, are two 
level which ferve to fet the line of o degrees truly hori- 
ontal. Hi is the perpendicular axis, whereby the inftru- 
acl may be tu ie . apa sie i face . ve inftru- 
ent may front index, 
iced to the sadness axis Hy, aaa wick pointe to an 
oppofite 
