the standpoint of facts as they exist to-day and of history as we
read it in the rocks and bowlders that we find distributed over the face
of the earth.

The whole northern part of the United States extending to a point south
of Cincinnati was at one time covered with a great ice-sheet, traces of
which are plainly visible to anyone who has made anything of a study of
this subject. The glaciers now to be seen in British Columbia and
Alaska, great as they seem to one viewing them to-day, are by comparison
with what once existed simply microscopic specks of ice. Glaciers, like
rivers, flow by gravity, following the lowest bed and lines of least
resistance; the difference being that in the one case the flow is rapid,
while in the other it is scarcely visible, except by measurement from
day to day. Before entering upon a description of the law that governs
the flow of glaciers, let us stop and give a little study to the
phenomena of water as exhibited when it is at the freezing point. Water
is such a large factor in the make-up of our globe and the air that
surrounds it that it becomes a very interesting and important study to
anyone who wishes to understand the phenomena of nature that are closely
related to it.

As all know, pure water is a compound of two gases, oxygen and hydrogen,
combined in the proportion of two atoms of hydrogen and one of oxygen.

Let us now study this fluid in its relation to heat. The reader is
referred to the chapters on heat in Vol. II., where it is stated that
heat is a mode of motion. It is also stated that heat is a form of
energy, and that energy is indestructible, that an unvarying amount of
it exists in some form or another throughout the universe. It is not
always manifested as heat or electricity, although both of these are
always in evidence as active agents of force. Much of the energy is
simply stored--all the time possessing the ability to do work or to be
converted into any of its known forms, such as heat, light, electricity,
or mechanical motion. A weight that is wound up has required a certain
amount of energy to elevate it to the position that it occupies. While
in its elevated position it possesses energy, although not active.
Energy in this form is called potential (possible) energy, and has the
power to do work if released. Active energy is called kinetic (moving)
energy, and the sum of these two energies is a constant quantity.

We will now study energy as it is related