a certain pull, through a certain space, a body is caused to clash
with a certain definite velocity against the earth. Heat is thereby
developed, and this is the only sense in which gravity can be said
to be converted into heat. In no case is the force which produces
the motion annihilated or changed into anything else. The mutual
attraction of the earth and weight exists when they are in contact
as when they were separate; but the ability of that attraction to
employ itself in the production of motion does not exist.

The transformation, in this case, is easily followed by the mind's
eye. First, the weight as a whole is set in motion by the attraction
of gravity. This motion of the mass is arrested by collision with
the earth; being broken up into molecular tremors, to which we give
the name of heat.

And when we reverse the process, and employ those tremors of heat to
raise a weight, as is done through the intermediation of an elastic
fluid in the steam-engine, a certain definite portion of the
molecular motion is destroyed in raising the weight. In this sense,
and this sense only, can the heat be said to be converted into
gravity, or more correctly, into potential energy of gravity. It is
not that the destruction of the heat has created any new attraction,
but simply that the old attraction has now a power conferred upon it,
of exerting a certain definite pull in the interval between the
starting-point of the falling weight and its collision with the earth.

So also as regards magnetic attraction: when a sphere of iron placed
at some distance from a magnet rushes towards the magnet, and has
its motion stopped by collision, an effect mechanically the same as
that produced by the attraction of gravity occurs. The magnetic