ider, is exceedingly important. We shall have
occasion to examine them with great care and minuteness under another
head, for upon them, more than any other portion of the arrangements,
depend not only the diffusion of heat, but also the distribution of
moisture.

Still another supply of heat, during the sudden changes, at least, is
produced by the action of terrestrial magnetism and electricity. Very
great progress has been made within a short period, in the investigation
of the nature of these agents. The identity, or at least intimate
association or connection of heat, light, electricity, and magnetism,
always suspected, has been in various ways, and by a variety of
experiments demonstrated. The influence of magnetism if distinct from
gravitation, is second only to that; and its agency in producing the
phenomena we are considering is primary and controlling. We will only, in
this connection, ask the reader to note the situation of the north
magnetic poles (for there are two of them); the manner in which the
isothermal lines _surround_ them; the fact that they are _poles of cold_,
_i. e._, that it is colder there than even to the north of them. We shall
recur to this part of the subject again.

Such, briefly considered, are the principal arrangements by which heat is
diffused over the earth.

Equally marked by infinite wisdom, and equally interesting and important,
are the arrangements by which moisture is distributed. Doubtless the
general belief is that this is a simple process; that water evaporates
and rises till it meets a colder stratum of atmosphere, and then condenses
and falls again; or that, according to the Huttonian theory, currents of
air of different temperatures mingle and equalize their heat, and the
aggregate mass when equalized in temperature is cooler, and therefore is
unable to hold as much moisture in solution as the most heated portion
had, and the excess falls in rain. But the process is by no means so
simple, nor is heat the sole or most powerful agent concerned in it.
Currents of air do not mingle, but stratify. Evaporation from the surface
of any given portion of the earth outside of the tropics does not alone
supply that portion with rain. _Vast and wonderful, coextensive with the
globe itself, and perfectly connected, is the machinery by which that
supply is furnished even to the most inconsiderable portion of its
surface._

Take your map of North America and note, in this respect, its