Mr. Thompson illustrates the effect of the friction of the earth's
surface on the eastward circulation of the air by a very simple
experiment with a pail of water. If we put into the pail grains of
any material a little heavier than water, and then give the water a
rotatory motion by stirring it, the grains ought, by the centrifugal
force imparted to them, to collect around the sides of the pail; but,
sinking to the bottom, they do in fact tend to collect at the centre,
carried inward by those currents which the friction of the sides and
bottom indirectly produces.

Thus Mr. Thompson's beautiful and philosophical theory completes
that of Halley, and explains all those apparent anomalies which have
hitherto seemed irreconcilable with the only rational account of the
trade-winds. The rainless calms of the tropics are explained by this
theory without that crossing and interference of winds which Lieut.
Maury supposes; for the secondary circulation returns as an
under-current toward the poles without reaching the tropics, and the
dry lower current of the principal circulation passes over the
tropical latitudes, in its gradual descent, before it reaches the
earth as the trade-winds.

These trade-winds, absorbing moisture from the sea, precipitate it
as they rise again, and produce the constant equatorial rains; and
these rains, doubtless, tend much more powerfully than the mere
unequal distribution of heat to direct the wind toward the equator;
for the fall of rain rapidly diminishes the pressure of the air and
disturbs its equilibrium, so that violent winds are frequently
observed to blow toward rainy districts. Thus, primarily, the unequal
distribution of heat, and, more immediately, the equatorial rains
cause the principal circulation of our atmosphere; and this