DIPPING. 
will be affeQed, a . is placed upon any part of the furface 
‘of that terrella. The poles are moft readily difcovered by 
‘trying where the ne of iron, or ee of needle, 
will fland erect upon the terrella; an is is generali 
feund to be two points which are dimesically op- 
is eon unknown ; Ww meter they are upon land or water ; 
or, in either cafe, whether we can come nigh tot oleae Yet, 
be thefe things as they may, it appears evident to that 
aecu ns, made as near to thofe m ade poles 
as pofitble, a“ a goo dip ing . is the fureft way to 
‘complete the magnetic theory of this globe, analogous to 
the method we purfue in examining = terrlia. But, as 
all the dipping needles which I have feen, appeared to me 
to be very ill Sang for the fea pete at leaft, I con- 
trived one upona different plan in 1764, and had it fecal 
before I left ae by Mr. Sifon, I have called it an 
universal magnetic > needle, or rete alse becaufe I can 
It ze the dip a litude, and the azimuth, 
ee the aleade: ms me. The 
ie e ate lize nearly as thofe ufed now 
for the compafles aie royal navy, and plays vertically upon 
its own axis, which has two conical points, fl:ghtly fupport- 
exed, at right 
c x inches diameter, filvered. and graduated to 
every half — upon which the needle fhews the dip, by a 
vernier, if you chocfe; and thie, for the fake of diftinGion, 
{ ll call ie circle of magnetic igen This brafs 
rcle of inclination, 
er vots, the one ane 
n the a 
lelogram. Thefe pivots are fixed 1 
an inch, an 
as to allow the circle of fielmaien and 
uch a diamete 
che paralle Ne ioneae - move freely round within it. This fe- 
cond circle I fhall call the pene meridian. It is not gra- 
duated, but has a fmali brafs weight fixed to the lower part 
of i Ae to keept upright ; and the circle itfelf is ferewed, at 
rig!t angles, into another circle, of equal internal diameter, 
of the fame thicknefs, and twice the breadth, which is fiver: 
ry bai ree. 
ely upon 85 
whole is contaired 
iu a neat ma with a glafs 
plate at ee and one on each fide, for ae two thirds 
down. art of the frame which contains the glafs 
‘lifts off ocealicnally. The whole box turns round upon a 
ftrong brals centre, fixed in a double plate of mahogary, 
glewed together crofs-wif-, to prevent its warping or fplit- 
and this again is {upported by three brals feet, fuch as 
eafily fl-p, if the veffel fhou n 
It has another {quare deat box t to ae it up in, to oprelaeve 
as mals ny when it is not w fe, 
sche uv ae aa eaucne | is pani elias as the inclina- 
tion, dip any time apparent from in{pection only ; 
and alfo the eo if the frame is turned round till the 
always i 
re s vertical plane, the edge of it ya evidently point iy 
‘upon the honzon the variation. eaft or welt. But at.fea, 
when there is not too much motion, ‘cel turn the ine 
L 
round till the vertical circle is in the ont the fun’s rays ¢ 
that is, till the fhadow of the one fide juft covers the 
other, and the edge of the circle of nclioation will then give 
the magnetic eee if the fun oo. etting ; but 
the azimuth a other times of th v3 and, the true 
amplitude or aah being found, in wn san way, the 
difference is the vari-tion. If the motion is confiderable, 
cbferve the extremes of the vibration, and take the mean for 
your Wig Fa amplitude, or azimuth. When the fun dees 
not fhine fo o bight 28 to a a fhadow, you can fet the brafs 
circle in a line with his body, if he is at all vifible, by your 
eye. ‘The principal ari at aime this com- 
pafs was, to cortrive a dippi So ccdl: which fhould be fuf- 
ficient for making obfervations at fea; needles, to 
be of ufe, muft be placed, 
amanner, as that al! their vibraticns fhall b 
true magnetic meridian, north and fouth, ac they 
u down, in any part of the world; the leatt dip, there- 
fore, is always in this magnetic line. But the only me 
of fetting a dipping-needle at fea, has hitherto been to place 
it in a line with the common compels. le; and this muft 
be very inaccurate, if they are at any confiderable di- 
ftance one from the other; or, if they were near, the two 
sake would influence one another, and neither of them 
ould be true: nay, fuppofing them for once to be properly 
paced in this line, the lealt motion of the fhip throws 
them 
have only one thing farther 
rve, that, as it is impoflible for human hands to make 
n we have two 
n t in this compafs, there 
of examining the obfervations. Take a 
good artificial ane and on the outfide of the compafs- 
a oe as end of it towards the needle, and, by moving 
magnet, you may thus guide the north ‘end of the 
eee ae the fouth, or vice erly -without opening your 
e magnet being then laid afide, the needle 
ions: but, a 
w reverfe 
nearly, I beli leve, as ie is poflible is any anime nt. to 
is ft. May sot a part of this {mall piel shgee be 
palit to the dire€tion 2f the magnetic influence, what 
ver that be, in the feel bar ? and, if fuch an experimen could 
be tried upon t e prefent azimuth compaffes, i is it not pros 
bable, that the vaio ix them would be at léaft as = fenfible 2. 
Query 2d. y not this be the caufe that = of the beft » 
of them will | “Tiffer a {mail matter from one another? 
Query 3d. Would the ends of the needle being made an- 
gular, inftead of the {quare Fam be, in fome Aes 
remedy for this {mall variation 
N.B. This inftvument requires a mot ‘exqilit oie 
ay anthip 5 
