SQ2 compass. 
pend upon the conffaivcy of the fun’s motion, and this 
appears to be a caufe fufficient to explain the phenomena, 
we Ihould (agreeably to Newton’s firft law of philofb- 
phizing) look no farther. As we therefore coniider, lays 
lie, the magnetic powers of the earth to be concentrated 
in the magnetic poles, and that thereis a diurnal variation of 
the magnetic needle, tliefe poles mutt perform a friiall 
diurnal revolution proportional to fuch variation, and re¬ 
turn again to the fame point nearly. Suppofe then that 
the fun in his diurnal revolution paffes along the north- 
-ern tropic, or along any parallel of latitude between it 
•and the equator, when he comes to that meridian in which 
the magnetic pole is fituated, lie will be much nearer to 
it, than in any other'; and in the oppolite meridian lie 
will of courfe he the fartheft from it. As the influence 
of the fun’s heat will therefore a£t molt powerfully at the 
Halt, and lefs forcibly at the greateft diltance, the mag¬ 
netic pole will confequently defcribe a figure fomething 
of the elliptical kind; and as it is well known that the 
greatell-heat of the day is fome time after the fun has 
paffed the meridian, the longell axis of this elliptical 
figure Vv'ill lie north-eafterly in the northern, and louth- 
ealterly in the fouthern hemifphere. Again, as the in¬ 
fluence of the fun’s heat will not from thofe quarters 
have fo much power, the magnetic poles cannot be moved 
back to the very fame point, from which they fet out; 
but to one which will be a little more northerly and eaft¬ 
erly, or more foutherly and ealterly, according to thehe- 
mifpheres in which they are fituated. The figures there¬ 
fore which they defcribe, may more properly be termed 
elliptoidal fpirals. 
In this manner the variation of the magnetic needle in 
the northern hemifphere may be accounted for. But 
with refpedt to the fouthern hemifphere we mult recolledt, 
that though the lines of declination in the northern he¬ 
mifphere have conllantly moved from welt to eall, yet in 
the fouthern hemifphere, it is equally certain that they 
have moved from ealt to weft, ever fince any oblerva- 
tions have been made on the fubjeit. Hence then the 
lines of magnetic declination, or Halleyan curves, as 
.they are now commonly called, appear to have a contrary 
motion in the fouthern hemifphere, to what they have in 
the northern ; though both the magnetic poles of the 
earth move in the fame direction, that is from weft to ealt. 
In the northern hemifphere there was a line of no va¬ 
riation, which had ealt variation on its ealtern fide, and 
weft variation on its weltern fide. This line evidently 
moved from weft to ealt during the two laft centuries; 
the lines of eaft variation moving before it, while the 
lines of weft variation followed it with a proportional 
pace. Thefe lines firft palled the Azores or Weltern 
Illands, then the meridian of London, and after a cer¬ 
tain number of years ftili later, they palled the meridian 
of Paris. But in the fouthern hemifphere there was 
another line of no variation, which had eaft variation on 
its weltern fide, and weft variation on its ealtern; the 
lines of eaft variation moving before it, while thofe of 
the weft variation followed it. This line of no variation 
firft paffed the Cape des Aiguilles, and then the Cape of 
Good Hope ; the lines of 5°, to 0 , 15 0 , and 20 0 , welt va¬ 
riation following, it, the fame as was the cafe in the 
northern hemifphere, but in the contrary dire< 5 lion. 
We may farther mention the idea of Dr. Gowin 
Knight, which was, that this earth had originally re¬ 
ceived its magnetifm, or rather that its magnetical powers 
had been brought into adtion, by a fliock, which entered 
near the fouthern tropic, and paffed out at the northern 
one. His meaning appears to have been, that this was 
the courfe of the magnetic fluid, and that the magnetic 
poles were at firft diametrically oppolite to each other. 
Though, according to Mr. Canton’s dodtrine, they would 
not have long continued fo; for from the intenfe heat of 
the fun in the torrid zone, according to the principles al¬ 
ready explained, the north pole mult have foon retired to 
the north-eaftward, and the fouth pole to the iouth-eaft- 
1 
ward. It is alfo curious to obferve, that on account cf 
the fouthern hemifphere being colder upon the whole 
than the northern hemifphere, the magnetic poles would 
have moved with unequal pace: that is, the north mag¬ 
netic pole would have moved farther iu any given time to 
the north-eaft, than the fouth magnetic pole could have 
moved to the fouth-eall. And, according to the opinions 
of the molt ingenious authors on this interelling fubjedt, 
it is generally allowed, that at this time the north mag¬ 
netic pole is confiderably nearer to the north pole of tne 
earth than the fouth magnetic pole is to the fouth pole 
of the earth. It may farther be added, that feveral in¬ 
genious fea-officers are of opinion, that in the wefteni 
parts of the Englifli channel the variation of the mag¬ 
netic needle has already begun to decreafe; having in no 
part of it ever amounted to twenty-five degrees. There are 
however other perfons who affert that the variation is ftili 
increafing in the Channel, and as far weftward as the fif¬ 
teenth degree of longitude and fifty-one degrees of lati¬ 
tude, at which place they fay that it amounts to about 
thirty degrees. 
The dip is a variation which differs from all the preced¬ 
ing. If a eompafs-needle,which is accurately balanced and 
fulpended, fo as to turn freely in a vertical plane, be ren¬ 
dered magnetical, the north pole will be depreffed, and 
the fouth pole elevated above the horizon : this property 
is called the inclination or dip of ike needle. As it is very 
difficult to balance a needle accurately, the poles are ge¬ 
nerally reverfed by a magnet, fo that its two ends may 
dip alternately, and the mean of the two is taken. This 
property was dilcovered by Robert Norman, about the 
year 1576. The dip is faid to be fubjedt to a variation. 
At this time in London it is about feventy-two degrees; 
from lome late obl’ervations it appears to diminifii about 
fifteen minutes in four years. The nature of this phe¬ 
nomenon is pleafingly illuftrated by carrying a fmall dip¬ 
ping-needle from one end of a magnetic bar to the other; 
when it Hands over the fouth pole, the north end of the. 
needle will be directed perpendicularly to it; as the needle 
is moved, the dip will grow lefs, and when it comes to 
the magnetic centre it will be parallel to the bar; after¬ 
wards the fouth end will dip, and the needle will Hand 
perpendicular to the bar, when it is diredtly over the 
north pole. 
Of the COMPASS VARIATION CHART. 
Dr. Halley having colledted a multitude of obfervations 
made on the variation of the needle in many parts of the 
world, was hence enabled to draw, on a Mercator’s chart, 
certain lines, fhewing the variation of the compafs in all 
thofe places over which they palled, in the year 1700, 
when l.e publilhed the firft chart of this kind, called the 
'variation chart. From the conilrudtion of this chart it 
appears, that the longitude of any of thofe places may be 
found by it, when the latitude and the variation in that 
place are known. Thus, having found the variation of 
the compafs, draw a parallel of latitude on the chart 
through the latitude found by obfervation; and the point 
where it cuts the curved line, whofe variation is the fame 
with that obferved, will be the Ihip’s place. A fimilar 
projedt of thus finding the longitude, from the known la¬ 
titude and inclination or dip of the needle, was before 
propofed by Henry Bond, in his treadle intitled The 
Longitude Found, printed in 1676. 
This method, however, is attended with two conlide- 
rable inconveniences : firft, That wherever the variation 
lines run eaft and welt, or nearly fo, this way of finding 
the longitude becomes itnperfedl, as their interledtion 
with the parallel of latitude mult be very indefinite; and 
among all the trading parts of the world, this imperfec¬ 
tion is at prefent found chiefly on the weltern coafts of 
Europe, between the latitudes of forty-five degrees and 
fifty-three degrees; and on the eaftern Ihores of North 
America, with fome parts of the Weltern Ocean and Hud- 
fon’s Bay, lying between the faid Ihores; but for the 
other 
