Inquiries eoiicefning the Magnetism of the Earth. 119 
includes a cojnous account of the principal experiments directed 
to this object. The essential part of the ’“conclusions deduced 
from his own investigiatioris, those of Graham, Canton, Hiorter, 
and others, is contained in the following extract. 
“ In addition to its annual movement, the needle has like- 
wise a sensible movement from day to day, and even from hour 
to hour. In Europe, it lies farthest to the east about 8 or 9 
o’clock in the morning, farthest to the west about 1 or 2 o’clock 
in the afternoon ; it next travels back eastward till about 8 or 9 
pkjlock in the evening, when it continues stationary for an hour 
or two, or else makes a slight recoil towards., the west ; during 
the night it commonly advances a little eastward, so that about 
8 in the morning it is found somewhat more easterly than it 
was the preceding et^ening. 
Besides this regular daily oscillation, there happen at times 
on a sudden, large extraordinary movements, in which the needle 
traverses, frequently with a shivering motion, an arc of seve- 
ral degrees on both sides of its usual position. Those appear- 
ances are seldom, perhaps never, exhibited, unless when the 
aurora borealis is visible ; and this disturbance of the needle 
seems to operate at the same time in places the most widely 
separated. Th® extent pf such e^^traOrdinary movements may, 
in less than twenty-four hp^^’s^ amount to 5° or 5^°? Jn such 
cases the disturbance is also communicated to the dipping 
needle, and so soon as the crown of the aurora quits the usual 
place (the points where the dipping needle produced would 
meet the sky,) that instrument moves several degrees forwardj^ 
and seems to follow it. After sucli disorders, the mean varia- 
tion of the needle is wont to change, and not to recover its pre^ 
vious magnitude till after a new and similar disturbance. 
The common daily movement about the summer solstice, is 
nearly twice as great as about the winter solstice : at the former 
season about |, at the latter | of a degree. There is likewise 
a regular monthly movement, such as that from the summer 
solstice to the vernal equinox, the needle travels westward; 
eastward from the vernal equinox to the summer solstice. 
“ The regular daily movement is smaller near the equator, 
and increases onward to the pole. The needle’s eastermost o^ 
westermost position does not happen at the same instant, nor 
