colder one to flow into the vacated space. Thus the air on the
equator, being heated by the vertical rays of the sun, rises, creating a
partial vacuum which the cold air from the poles rushes equator-ward to
fill, the warm air moving toward the poles to restore the balance. Thus
at a few degrees north of the equator the upper stratum of air will
always be found to be travelling northward. And it continues so to do
until it reaches the vicinity of the thirtieth parallel of latitude,
when, having lost most of its heat by constant exposure to open space,
it becomes cold enough to descend, taking the place of the polar
current, which meanwhile has been warmed by passing over the temperate
zone. The equatorial current, though it has descended to the surface of
the earth, still makes its gradual way northward, as well as local
circumstances will permit, in order to replace the southward-flying
polar current; and by the time that it reaches the Arctic circle, it has
again, by contact with the earth, become the warmer of the two currents,
when it once more rises into the upper regions of the atmosphere, to
descend no more until it reaches the vicinity of the pole, when it
sinks, and at the same time turns southward as the polar current. And
the same thing happens in the southern hemisphere. Thus in each
hemisphere we have two great atmospheric currents--one flowing from the
pole to the equator, and the other flowing from the equator to the pole.
The lower current, or that which sweeps along the surface of the earth,
meets with so many disturbing local influences that it is frequently
deflected greatly from its proper course, sometimes so much so that its
course becomes completely reversed for a time; but in the upper regions
of the atmosphere these disturbing influences are very little if at all
felt. Now, if I have succeeded in making this plain to you, you will
readily understand that where the top of the lower current and the
bottom of the upper current touch each other there will be so much
friction that a neutral or `calm belt' will occur in which the air will
be motionless. And it is in this calm belt--which occurs between the
altitudes of three thousand and twelve thousand feet above the earth's
surface--that I propose we should take refuge to-night."

The professor's small audience duly expressed their thanks for the
extremely interesting lecture to which they had just been treated, and
then the party retreated t