variations of the Magnetic Forces. 337 
that the pressure of dry air has but°one ‘maximum’ and one mine 
imum during the year,—the former in the wiuter-and the, latter 
in the summer. The tension of the at mospheric vapor, ou the 
other hand, attains its minimum in the winter and maximum in 
the summer. From the combination of these two pressures there 
results two maxima and two minima of actual pressure, as pre= — 
camped stated. There i Isa minimum i in the spring because that 
tity of vapor is not yet very consi etabl e. We find traces of. a 
second minimum in autumn because the quantity of aqueous 
— diminishes rapidly, while the prageure of ‘dry air increases 
8 
It must accordingly be admitted to be. highly probable that 
: the annual variations of horizontal magnetic intensity are attribu- 
| table to the combined operation of the same two general antag 
Pe onistic causes as the diurnal variations, "iF Race of tem- 
Perature and variations of humidity. It must also be admitted 
that the existenice of such a connection a these diarnal and 
a ‘nnual maguetie and meteorological phenomena is a-necessary 
anlerence from the Thermal Theory of Terrestrial Magnetism ; 
anc this theory finishes a rational’and consistent explana- 
on of t the laws of the magnetic variations. 
t Variations of the Vertical Magnetic Intensity. 
urve showing the annual variations of the 
magne imvansity vat Phila delphia for the years [541 to 
1815, we ese thes the general law is that the intensity Is greatest 
or July, and least about. December or January. To 
this geen law. anfexce tion occu “a the year 1841—1he force 
hg, instead of rom the first of the year until 
and then j wacreaeing until Rorember after which there is a 
- accordance with the general law. Now it will be 
hat agreenbly to the theory under consideration, the 
= magnetic*force is dependent upon the differences of tem- 
perature between the station of the needle and all places situated 
to the north or south of this station, within the circle of sensible 
‘Magnetic ac action. We have therefore to enquire whether these 
erences obaatve the same law of variation as the vertical mag- 
hetic intensity ; and also whether there is any exception to the 
 _«:Seneral law, corresponding with that above mentioned, during the 
—-Year 1841, The observations which I have obtained snited to 
_ this inquiry, (viz., the mean monthly temperatures at Wash- 
ington, Newtown, Port Carbon, and Silver Lake, for 1841 and a 
of 1842, and at Trenton for 1842) furnish results which ac- 
_ for the most part, with the theory. ut as these results 
‘ate confined to one or two years, no certain conclusion can be 
os ‘Sram, Vol. X, No. 30,—Nov., 1859. 48 
ee 
ze. 
ee aly, a 
