turn-table such as is used about railway stations to reverse the
position of a locomotive. If the observer will stand in the centre of
such a table while it is being turned round he will perceive that his
body is not swayed to the right or left. If he will then try to walk
toward the periphery of the rotating disk, he will readily note that
it is very difficult, if not impossible, to walk along the radius of
the circle; he naturally falls behind in the movement, so that his
path is a curved line exactly such as is followed by the winds which
move toward the equator in the trades. If now he rests a moment on the
periphery of the table, so that his body acquires the velocity of the
disk at that point, and then endeavours to walk toward the centre, he
will find that again he can not go directly; his path deviates in the
opposite direction--in other words, the body continually going to a
place having a less rate of movement by virtue of the rotation of the
earth, on account of its momentum is ever moving faster than the
surface over which it passes. This experiment can readily be tried on
any small rotating disk, such as a potter's wheel, or by rolling a
marble or a shot from the centre to the circumference and from the
circumference to the centre. A little reflection will show the
inquirer how these illustrations clearly account for the oblique
though opposite sets of the trade winds in the upper and lower parts
of the air.

The dominating effect of the tropical heat in controlling the
movements of the air currents extends, on the ocean surface, in
general about as far north and south as the parallels of forty
degrees, considerably exceeding the limits of the tropics, those lines
where the sun, because of the inclination of the earth's axis, at some
time of the year comes just overhead. Between these belts of trade
winds there is a strip or belt under the region where the atmosphere
is rising from the earth, in which the winds are irregular and have
little energy. This region of the "doldrums" or frequent calms is one
of much trouble to sailing ships on their voyages from one hemisphere
to another. In passing through it their sails are filled only by the
airs of local storms, or winds which make their way into that part of
the sea from the neighbouring continents. Beyond the trade-wind belt,
toward the poles, the movements of the atmosphere are dependent in
part on the counter trades which descend to the surface of the e