® 
DIPPING. 
Sppofite line on the horizontal plate K, when the inftru- 
ya is pala half round. L, L, are four adjufting 
fcre et the inftrument properly. One of thefe 
aang is ‘hi d behind te circle. M,M, M, M, are fcrews 
which hold the Saati to prevent the difturbance of 
ihe needle by th 
he 66th rolume of the Philofophical Tranfations con« 
tains an account of the meteorological inftraments ufed at the 
Royal ere s houfe, by the Hon. Henry Cavendith ; and 
in sae eo of this account, which treats of the connie: 
tion and management of the dipping J Mr. Cavendifh 
expres himfel in a ae ma 
In nd 
inftrume the ends of the a of Ai — 
r balancing it, are the fame 
as in-the dipping-needle def{cribed in the Phil. a vol. 
62. It is alfo made by the fame artift, Mr. Nai 
‘‘ Tt may be feen in the Meteorological Journal, an the 
dip was obferved firft with the front of the inftrument to 
the weft, and then to the eaft; after which the poles of 
the needle were reverfed, and the dip obferved both ways 
as se » The reafon of this is, that the mean of the ob- 
uations, differs very little from 
the ne cdl is not well balanced, and even 
though a great many other errors are committed in the con- 
pobre of the iftrament "provided the needle is made 
equa as before ; (it is 
eafy to fee whether the needle is made equally magnetical 
after the poles are reverfed as before, by counting the num 
ber of vibrations which it makes i ~ a aes ;) and that the 
difference of the obferved dip, in thefe four fituations, is 
not very great, as will appear pie 7 following confider- 
ei 
" irk, let fig. 28. be a front view of the needle; ABa 
line parallel to the dire&tion of magnetifm therein; and 
tle parts mn and mg be equal. When the 
turned haif way round, fo that the contrary face the 
needle is prefented towards us, the edge, ADB, which is 
now loweft, will become uppermoft, and the centre of gra- 
vity will be i in that fituation in which the point # now is; 
me, as if the centre of gravity was 
the centre of gravity is at m, the dip will be 
ve ry n early as much too great in the prefent ftate of the 
elie: as it will be too little when the ool are reverfed. 
‘Lherefore, the mean of the obferved dips in thefe four fi- 
tuations - be very nearly the fame as if the needle was 
truly balan 
“ Secu. if the planes on which the axis rolls are not 
horizontal, the dip will be very nearly as much greater than 
it would otherwife be, when one face is turned to the weit, 
se is very nearl 
But if 
as it is lefs when the other is; for if if thefe planes dip to~ 
iaftrument is fet, to remain unaltered. Confequently, 
ean of th obfervations wiil be very nearly thé fam 
as if they were placed truly horizonial. 
‘¢ Thirdiy, by the fame method ie reafoning it appea 
that the mean of the two abovementioned eblervatio-s will 
be not at ail altered, though the line, joining the mark on 
that end of the needle by which we obferve, with the axis 
of motion, is not parallel to the dire€tion of magnetifm in 
e needie; that 1s, thou e r not coincide 
with the point A or B, or though the line | comie | the two 
divifions of go° is not perpendicular to the horizon, 
nel are with it. If, indeed, the axis of motion is not 
in the fame mile plane with: a eal of the divided 
circle, the r proceeding from will not be com 
penfated by pe method of obi: san unlefs ae se ae 
the needle ar a ule of. This, however, is o 
on- 
eafy to examine whether they are in oe 
ane horizontal ola or not. 
* But the error which is moft dificult to be avoided 
eve ivin 
ie liberty i in ae notches by which it is lifted up and 
hofe planes bear againfta part of the axis dif- 
se about ,.3,th or =4th of an inch from their ufual point of 
bearing. Now, I find, that when the axis is confined fo as to 
have none of this liberty, and when care is taken, by previoufly 
making the needle ive at nearly theright dip, that it fhall 
vibrate in very {mall arches when let iy peat on the planes ; 
that then, if the peal is ean ne: 
of times, it _ ern fett 
each time, at 
iy fenfible ; ; oe if ts is nae ‘fo co 
a difference of 20' in the dip, seconlag as different parts of 
he axis reft on the planes ; and that, though care is taken to 
free the axis and planes from duit as perfectiy as_poffible, 
which can be owing only to fome aA ae In the axis. 
e needle vibrates in arches or more 
t down on the p anes, hee will frequently 
be as error inthe dip. It is true, that the part of 
the agate planes, which the axis refts on when the vibrations 
are ft ill be a little different according to the point 
which the needle ftood at before it w down; whic 
will make a {mall difference in the dip as fhewn by the di- 
vided circles, when only one end of the needle is obferved, 
d and polifhed with great accuracy ; but it 
mott wae rose from the axis flipping in the iarge vi- 
ratio to ate Davee bear a 
gine too, that t 
more fo, than ny which has been 
We fhail now {ubjoin a ftatement of eee carefully 
made 
