h passes round its perihelion, the kinetic energy is used up in
assisting the earth to overcome the attraction of the sun. Thus there is
this oscillation from kinetic to potential, and from potential to
kinetic, year in and year out, as the earth performs its cycle round its
central body the sun.

Professor Tait, in the work referred to in the previous Article, gives
examples of kinetic forms of energy under the following heads--

1st. Winds.
2nd. Currents of Water.
3rd. Hot Springs and Volcanoes.

It can be readily seen that winds are a form of energy, as we have
innumerable instances of the power and energy which they exert.
Advantage is taken of that kinetic energy by means of windmills, in
which the energy of the wind is imparted to the revolving sails, and
thence to the machinery, various forms of mechanical work being the
result, as, for example, the grinding of corn, or the pumping of water.
The pressure or energy of winds has even been calculated, the following
figures being examples--

VELOCITY IN MILES PER HOUR. FORCE IN LBS. PER SQ. FOOT

1 mile. .005 lb. per sq. foot.
5 " .123 " " "
10 " .496 " " "
15 " 1.11 " " "
20 " 1.98 " " "
30 " 4.5 " " "
40 " 7.9 " " "
50 " 12.5 " " "

In the case of currents of water, whether they are in the form of river
currents or ocean currents, as has already been pointed out in the
previous article, the question of potential energy, or energy of
position, is associated with their kinetic energy. Water is taken at a
certain elevation, and then allowed to fall to a lower level, and in its
fall from the high level to the lower level, its kinetic energy is used
to drive mill-wheels, and thus work is done, the kinetic energy of the
water being transformed into the motion of the machinery. This machinery
may be used to work a dynamo, and thus electric light may be generated,
or it may drive an electric motor which may perform all sorts of
mechanical work. The great underlying principle of either kinetic or
potential energy rests in the fact, that wherever we have energy of any
kind or sort, whether it be associated with water, wind, or Aether,
there we have the capacity to do work, the amount