the
ethereal medium surrounding the earth, at perihelion and aphelion, being
least at the former, and greatest at the latter.

As a modifying and interfering cause in the action of the vortices, we
must mention the great natural currents of the atmosphere, due to the
earth's rotation.

It is considered that the sun is the principal cause of these great
currents. By elevating the surface atmosphere of the equator, a lateral
current is induced from the north and south; but on account of the
enlarging circles of latitude, their direction tends more from the
north-east and south-east. These currents are usually called the trades.
Without disputing the correctness of this, it may be doubted whether the
whole effect is due to the sun. As this principle affects the ocean
likewise, it is necessary to look into it; and in order to simplify the
question, we will first suppose our globe covered entirely by the ocean,
without any protuberant land.

Let us assign a uniform depth of ten miles to this ocean. In the Fig.
following, the two circles will represent the surface and bottom of the
ocean respectively. The axis of rotation is thus represented by the line
PP'. Let us consider two particles of water at m and n, as feeling the
influence of this rotation; they will, of course, be both urged towards
the equator by the axifugal force. Now, every particle in the ocean
being also urged by the same force, it might be supposed that after a
protuberant mass of water had accumulated at the equator EE', the whole
ocean would be in equilibrium. This would not follow. The particle at m
is urged by a greater force than n; consequently the particle at n is
overborne by the pressure at m. Considering both in the same direction,
yet the particle at n must give way, and move in the opposite direction.
Just as the heaviest scale of the balance bears up the lightest,
although both gravitate towards the same point. This is so self-evident
that it would seem unnecessary to dwell upon it, had not the scientific
world decided that the rotation of the earth can cause no currents
either in the atmosphere or in the ocean.

[Illustration: Fig. 22]

The axifugal forces of the two particles m and n are directly as the
lines Mm and Nn, and if the gravitating forces were also as the radii Tm
and Tn, no motion would be produced. Admitting even the Newtonian law to
be rigidly exact, the earth cannot be considered a homogeneous globe,
but, on the contrary,