COM 
ffxty-five revolutions the difference is fcarcely fenfible. 
That there is any difference of this kind, arifes from 
hence, that the impulfe by which the diurnal motion was 
impreffed'on the earth, was given to the external parts, 
and from thence in time communicated to the internal; 
but fo as not yet perfectly to equal the velocity of the firil 
motion impreffed on the fuperficial parts of 4 tlie globe, 
and ltill prelerved by them. As to the precife period, 
obfervations are wanting to determine it, though the au¬ 
thor thinks we may reafonably conjecture that the Ameri¬ 
can pole has moved weltward forty-lix degrees in ninety 
years, and that its whole period is performed in about 700 
years. But Whilton as well as Euler has controverted 
and cenfured.this theory of Dr. Halley. 
There is alfo a fmall variation of the variation of the 
magnetic needle, amounting only to a few minuses, of a 
degree in the fame place, at different hours of the fame 
day, which is only difcoverable by nice obfcrvation, 
and is termed the diurnal variation of the compafs. Mr. 
George Graham made leveral oblervations of thi kind in 
3722 and 1723, profeffmg himlelf altogether ignorant of 
the caufe of the phenomena he oblerved. About the 
year 1750, Mr. Wargentin, lecretary of the Swedifh aca¬ 
demy of fciences, took notice both of the regular diurnal 
variation of the needie, and alfo of its being diilurbed at 
the time of the aurora borealis, as recorded in the Philof. 
Tranf. vol.47, p. 126. In 1756, Mr. Canton commenced 
a feries of oblervations, amounting to near 4000, with an 
excellent variation-compafs, of about nine inches diame¬ 
ter. The number of daj's on which thefe obfervations 
were made, was 603, and the diurnal variation on 574 of 
them w'as regular, lo as that the ablolute variation of the 
needle weltward was increafing from about eight or nine 
o’clock in the morning, till about one or two in the af¬ 
ternoon, when the needle became ftationary for fome 
time; after that, the ablolute variation weltward was de- 
creafing, and the needle came back again to its former 
fituation, or nearly fo, in the night, or by the next morn- 
ing. The diurnal variation is irregular when the needle 
moves ilowly ealtward in the latter par t of the morning, 
or weltward in the latter part of the afternoon ; alfo w’hen 
it moves much either way after night, or fuddenly both 
ways in a flvort time. Thefe irregularities kldom happen 
more than once or twice in a month, and are always 
accompanied, as far as Mr. Canton oblerved, with an 
aurora borealis. On this Mr.. Canton lays down, and 
evinces by experiment, the following principle, viz. that 
the attractive power of the magnet (whether natural or 
artificial) will decreafe while the magnet is heating, and 
increafe while it is cooling. He then proceeds to account 
for both the regular and irregular variation. It is evi¬ 
dent, he lays, that the magnetic parts of the earth in the 
north, on the euft-fide, and on the welt lide of the magne¬ 
tic meridian, equally attract the north end of the needle. 
If then the ealtern magnetic parts be heated falter by the 
fun in the morning, than the weltern parts, the needle 
will move weltward, and the ablolute variation will in¬ 
creafe: when the attracting parts of the earth on each 
fide of the magnetic meridian have their heat increafing 
equally, the needle will be ftationary, and the ablolute 
Variation will then be greateft; but when the weltern 
magnetic parts are either heating falter, or cooling (lower, 
than the ealtern, the needle will move ealtward, or the 
ablolute variation will dtcreafe; and when the ealtern 
and weltern magnetic parts are cooling equally fall, the 
needie will again be ftationary, and the ablolute variation 
will then be leaft. By this theory, the diurnal variation 
in the fumhier ought to exceed that in winter; and ac¬ 
cordingly it is found by obfervation, that the diurnal va¬ 
riation in the months of June and July is almolt double 
of that in December and January. 
The irregular diurnal variation mull arife from fome 
other caufe than that of heat communicated by the fun; 
and here Mr. Canton has recourie to lubterranean heat, 
which is generated without any regularity as to time, 
PASS. ' 891 
and which will, when it 'happens iri the north, affeft the 
attractive power of the magnetic parts of the earth on 
the north end of the needle. That the air nearelt the 
earth will be molt warmed by the heat of it, is obvious; 
and this has been often noticed in the looming, before 
day, by -means of thermometers at different diftances 
from the ground. Mr. Canton has annexed to his paper 
on this lubjeCl, a complete year’s obfervations; from 
which it appears, that the diurnal variation increales 
from January to June, and decreases from June to De¬ 
cember. See Philof. Tranf. vol. 51,-p, 398. 
With refpeCf to the influence of the aurora borealis on 
the compafs needle, Meffrs. Wilcke and Van Swinden 
have ftiewn it to be fo evident, fo general, and fo con- 
ftant, that no one, who examined ! he affeCtions of the 
one and the other with attention, could have any doubts 
on the fubjeff. It remained, however, for Mr. Dalton, 
in his Meteorological Obfervations, publffhedin 1793, to 
give a complete and fatisfaclory account of this con¬ 
nexion. From various-obfervations he has demonftrated, 
1. When the aurora appears to rife only about 5 0 10', or 
15° above the horizon, the needle is very little dilturbed, 
and often infenfible. 2. When it riles up to the zenith, 
and pafles -it, there never fails to be a confiderable dif- 
turbance. 3. This dillurbance confifts in a regular of- 
cillation of the compafs-needle, fometimes to the eaft- 
ward, then to the weltward, of the mean daily pofition, 
in fuch fort, that the greateft excuriions on each fide are 
nearly equal, and amount, at Manchefter, to about half a de¬ 
gree 011 each fide. 4. When the aurora ce.ifes, or focn 
after, the needle returns to its former ftation. From 
thele fails alone, fays'Mr. Dalton, independent of other 
obfervations, we cannot avoid inferring, that there is 
fomething magnetic in the higher regions of the atmo- 
fphere, that has a (hare in guiding the needle; and that 
the fluctuations of the needle, during the aurora, are oc- 
cafioned by fome mutations that then take place in this 
magnetic matter in the incumbent atmofphere. 
It has been oblerved, that different needles, efpecially 
if touched with different loadltones, will differ a few mi¬ 
nutes in their variation. Dr. Lo rimer (in the Supp. to 
Cavallo’s Magnetilm) adduces fome ingenious obferva- 
tions on this lubjeCt. It muff be allowed, fays he, ac¬ 
cording to the oblervations of feveral ingenious gentle¬ 
men, that the coileCiive magnetilm of this earth arifes 
from the magnetilm of all the ferruginous bodies con¬ 
tained in it, and that the magnetic poles fhould therefore 
be confidered as the centres of the powers of thole .mag¬ 
netic fubftances. Thefe poles mult therefore change 
their places according as the magnetilm of fuch fub- 
ftances is affedfed; and if with Mr. Canton .we allow, 
that the general cauie of the diurnal variation arifes from 
the fun’s heat in the forenoon and afternoon of the fame 
day, it will naturally occur, that the fame caufe, being 
continued, may be lufficient to produce the general va¬ 
riation of the magnetic needle for any number of years. 
For we mult conlider, that ever fince any attentive ob¬ 
fervations have been made on this fubjecl, the natural 
direilion of the magnetic needle in Europe has been con- 
ftantly moving, from w'eft to eaft, and that in other parts 
of the world it has continued its motion with equal co;i- 
ftancy. 
As we muff therefore admit, fays Dr. Lorimer, that 
the heat in the different feafons depends chiefly on the 
fun, and that the months of July and Auguft are com¬ 
monly the hotteff, while January and^February are the 
coldelt months of the year; cind that the temperature of 
the other months falls into the relpeffive intermediate de¬ 
grees ; fo we mult confider-the influence of heat .upon 
magnetilm to operate in the like manner, viz. that for a 
fliort time it fcarcely manifelts itfelf; yet in the courfe 
of a century, the conftancy and regularity of it becomes 
fufficiently apparent. It would therefore be idle to fup- 
pol'e, that fuch an influence could be derived from an un¬ 
certain or fortuitous caufe. But if it be allowed to de- 
