three hundred feet high, which are sometimes constructed, the
uprush is at the speed of a gale.

Whenever the air next the surface is so far heated that it may
overcome the inertia of the cooler air above, it forces its way up
through it in the general manner indicated in the chimney flue. When
such a place of uprush is established, the hot air next the surface
flows in all directions toward the shaft, joining the expedition to
the heights of the atmosphere. Owing to the conditions of the earth's
surface, which we shall now proceed to trace, these ascents of heated
air belong in two distinct classes--those which move upward through
more or less cylindrical chimneys in the atmosphere, shafts which are
impermanent, which vary in diameter from a few feet to fifty or
perhaps a hundred miles, and which move over the surface of the earth;
and another which consists of a broad, beltlike shaft in the
equatorial regions, which in a way girdles the earth, remains in
about the same place, continually endures, and has a width of hundreds
of miles. Of these two classes of uprushes we shall first consider the
greatest, which occurs in the central portions of the tropical realm.

Under the equator, owing to the fact that the sun for a considerable
belt of land and sea maintains the earth at a high temperature, there
is a general updraught which began many million years ago, probably
before the origin of life, in the age when our atmosphere assumed its
present conditions. Into this region the cooler air from the north and
south necessarily flows, in part pressed in by the weight of the cold
air which overlies it, but aided in its motion by the fact that the
particles which ascend leave place for others to occupy. Over the
surfaces of the land within the tropical region this draught toward
what we may term the equatorial chimney is perturbed by the
irregularities of the surface and many local accidents. But on the
sea, where the conditions are uniform, the air moving toward the point
of ascent is marked in the trade winds, which blow with a steadfast
sweep down toward the equator. Many slight actions, such as the
movement of the hot and cold currents of the sea, the local air
movements from the lands or from detached islands, somewhat perturb
the trade winds, but they remain among the most permanent features in
this changeable world. It is doubtful if anything on this sphere
except the atoms and molecules of matter have varied as litt