COMPASS. 
attached as to render this hiftrument as familiar to the 
people afhore, as it is at fea. 
The nature and the caufe of the qualities of the mag¬ 
net have, at all times, been fubjeds of contemplation 
among the Chinefe. Their theory, in this inftance, as 
in many others, is the reverfe of that of European phi- 
iofophers. It is obvious that while the magnetic needle, 
fuipended by its centre, points at one extremity to the 
north, it necelfarily looks, at the other, to the fouth; 
but each retains its own polarity; and, if turned round 
by force, will refume, when left at liberty, its original 
Ration oppofite its refpedive pole. Thus the power, 
which principally attrads the needle, may be fuppofed 
to refide toward either or both portions of the earth. In 
JSufoperit has been thought that the needle has its chief 
tendency to the north pole; but in China the fouth alone 
is confidered as containing the attractive power. The 
Chinefe name of the compal's is ting-nan-cbing, or needle 
pointing to the fouth ; and a diftinguilhing mark is fixed 
on the magnet’s fouthern pole, as in the European com- 
pafs it is upon the northern one. The emperor Caung- 
ftiee, grandfather to the late emperor Tchien-lung, who 
was in the habit of committing to paper his oblervations 
on a variety of fubjeds, and who, having encouraged 
learned milfionaries at his court, had not been inattentive 
to their philofophical opinions, writes on this occafion : 
I have heard Europeans fay, that the needle obeys the 
north. In our oldeft records it is faid, that it turns to 
the fouth ; but as neither have explained the caufe, I fee 
little to be gained in adopting one opinion in preference 
to the other. The ancients, however, are firft in date; 
and the farther I proceed, the more I am convinced of 
their knowledge of the operations and mechanifin of na¬ 
ture. Moreover, as all adlion grows languid, and nearly 
is fufpended towards the north, it is lels likely that the 
virtue, which gives motion to the magnetic needie, fhould 
proceed from that quarter.” 
Of the VARIATION of the COMPASS. 
The compafs was many years in ufe before it was 
known in anywife to deviate from the poles of the world. 
About the middle of the fifteenth century, fo confident 
were fome perfons that the needle invariably pointed due 
north, that they treated wdth contempt the notion of the 
variation, which about that time began to be l’ufpeded. 
However, careful oblervations foon difeovered that in 
England, and its neighbourhood, the needle pointed to 
the ealtward of the true north line; and the quantity of 
this deviation being known, mariners became as well fa- 
tisfied as if the compal's had none ; becaufe the true couri'e 
could be obtained by making allowance for the true va¬ 
riation. This variation is different in different places, on 
land as well as at fea, and is continually varying in the 
fame place. For inltance, the variation is not the fame 
in London as at Paris, or at the Cape of Good Hope ; 
and the declination at London, or at any other place, 
is not the fame now as it was twenty years ago. This 
variation is always reckoned from the north; that is, if 
the north end of a needle vary to the eaft of the north, 
the variation is faid to be eafterly; and if it vary to the 
well, the variation is faid to be wefletiy: hence, in fea- 
Ltnguage, this variation is ufually called north-eafung, or 
.nortb-nvefling. 'I he uncertainty of the quantity of this 
variation in different parts of the world, has been a great 
impediment to the perfeding of navigation ; and philofo- 
phers have earneftly endeavoured to inveffigate its caufe, 
and, if pofiible, to correct the errors it occaiions. 
The variation of the compafs is faid to have been firlt 
difeovered by Columbus, the latter end of the fifteenth 
century. But the firft perfon who difeovered that it was 
real, and was the fame to all needles in the fame place, is 
generally allowed to be Seballian Cabot. This was about 
the year 1497. After the variation was difeovered by 
Cabot, it was thought, for a long time, to be invariably 
■the fame at the fame places in all ages; but Mr. Gelli- 
Vol. JV. No, 247. 
8 P 9 
brand, about the year 1625, difeovered, that it had a 
variation of the variation, or was different at different 
times in the fame place. From fucceflive obfervations 
made afterwards, it appears, that this deviation was not 
a conftant quantity, but that it gradually diminilhed, 
and at laft, about 1657, it was found, that the needle 
pointed due north at London, and has ever fince been in- 
creafing to the weft ward of the north. So that in any 
one place the variations have a kind of libratory motion, 
traverfing through the north to unknown limits ealtward 
and weftward. The prefent variation at London is about 
two points, or twenty-three degrees weft of the north. 
Dr. Halley fuppofed, that the earth has within it a 
large magnetic globe, not fixed within to the external 
parts, having four magnetic poies, two fixed and two 
moveable, and by this he has endeavoured to account for 
the phenomena of the needle. His application of this 
theory to facts is in many reipeds inadequate; and Mr. 
Euler has fhewn, that he can with two magnetic poles 
placed on the furface of the earth, account for ail the 
phenomena, as well as Dr. Halley with four; but his 
theory has alfo various imperfedions. This variation of 
the needle may be illuftrated by placing feveral touched 
needles round a magnetic bar; as delineated in the en¬ 
graving at fig. 10. Now, if the earth be a great mag¬ 
net, or if it have only a magnetic atmofphere, it is clear 
from this experiment, that magnetic needles placed on 
its furface would have different diredions in different 
places, which is conformable to experience; and the ap¬ 
parent irregularities in the variation of the needle mult 
be occafioned by the fituation of the magnetic poles of. 
the earth, or of magnetic matter near its furface. There 
are many inftances on record of very extenfive magnetic 
rocks, which affed the needle to very confiderable dis¬ 
tances. The ifland of Elbe in the Mediterranean is a 
very remarkable inftance of this. The ifland of Gan hay 
alfo, on the weft of Scotland, has rocks which affed the 
needle at a great diftance. A fimilar effed is obferved 
near the Ferro iflands in the North Sea ; the compafs has 
no determined diredion when brought on (hore. In Hud- 
for.’s Straits, in latitude 63°, the needle has hardly any 
polarity. Bouguer obferved the fame thing in Peru. Nay, 
we believe that almoft all rocks, efpecially of whin or 
trappe ftone, contain iron in a proper ftate. All this re¬ 
fers only to the thin cruft through which the human eye 
lias occafionally penetrated. Of what may be below we 
are ignorant; but when we fee appearances which tally 
fo remarkably with what would be the’ effeds ot great 
maffes of magnetical bodies, modifying the general and 
regularly p'rogreffive adion of a primitive magnet, whole 
existence and motion is inconfiftent with nothing that we 
know of this globe, this manner'of accounting for the 
obferved change of variation, has all the probability that 
we can defire. In Italy, father de la Torre obferved, that 
during a great eruption of Vefuvius, the variation was 
fixteen degrees in the morning, at noon it was fourteen 
degrees, and in the evening it was ten degrees, and that 
it continued in that ftate till the lava grew lb dark as no 
longer to be vilible in the night; after which it llovvly in- 
creafed to thirteen and a half, where it remained.. Daniel 
Bernoulli found the needle change its polition forty-five 
minutes by an earthquake. ProfelTor Muller, at Man- 
heim, obferved that the declination of the needle in that 
place was greatly affeded by the earthquake in Calabria. 
Such ftreams of lava as flowed from Hecla in the fait 
dreadful eruption, muft have made a transference of mag¬ 
netic matter that would conliderably affed the needie. 
But no obfervations feem to have been made on the occa¬ 
fion ; for we know that common iron-ftone, which has no 
effed on the needle, will, by mere cementation with any 
inflammable fubltance, become magnetic. In this way 
Dr. Knight fometimes made his artificial loadftones. We 
have faid fo much chiefly with the view of cautioning our 
readers againft too fanguine expeditions from any pre¬ 
tentions to the folution of this great problem. We may 
