ame latitudes. Very great diversities exist in the "annual
mean" as well as the "mean" of the different seasons. Accurate
observations at many points have enabled men of science to demonstrate
this by drawing isothermal lines (_i. e._, lines of equal average annual
heat) from point to point around the earth, which show at a glance these
differences. The annexed cut is a polar projection of the isothermal lines
of the northern hemisphere, as far down as the tropic, copied from
Kaemtz's Meteorology. The dotted lines show the parallels of latitude, the
dark lines the isothermal lines, or lines of equal annual average
temperature. The reader is desired to observe how rarely they correspond
with the parallels of latitude, and how they fall below in a few
instances, and in others with great uniformity rise almost to the pole.

Take, for example, the isothermal line of 0 or zero--that is, the line
where the mean or _average_ height of the thermometer _for the year_ is at
zero. At Behring's Straits this line is a little below the Arctic circle,
or the parallel of 66.30 north latitude. Passing east over North America,
it descends into Canada, almost to Lake Superior, and to about the 50th
parallel: that is to say, it is on an average during the year as cold on
our continent at the 50th parallel as it is near Behring's Straits at the
65th parallel. Passing east, the line of zero rises again over the
Atlantic Ocean until, in the meridian of Spitzbergen, it reaches, within
the Arctic circle, up almost to the 75th parallel. So, too, the isothermal
of 5 deg. below zero, which is below the 60th parallel in Siberia, rises in
the North Sea, above Behring's Straits, to the parallel of 75 deg., descending
on the continent in North America to the 55th parallel, and rising again
almost to the pole at Spitzbergen, to descend again in Siberia, while the
isothermals of 10 deg. and 15 deg. below zero, which in North America are but just
above the latitude of 60 deg. and 75 deg. respectively, ascend abruptly
_surrounding the magnetic pole_, and _falling short of the geographical
one_. Let this projection of the lines of equal temperature, and
particularly the situation of the magnetic poles, be studied well, for we
shall recur to it hereafter in illustration of many important portions of
our subject.

It is apparent from these facts, and were it necessary might be rendered
still more so by referring to others, that other causes operate in the
di