substance, in obedience to the summons. Mobile water runs down from a
higher to a lower level because the latter is nearer to the earth's
centre than the former, and as it falls it pushes before it such minor
obstructions as are unable to resist the influence of its weight. The
float-boards of the mill-wheel are of this nature; they are striving to
uphold the water by means of the rubbing and friction of the apparatus
that is mechanically connected with the axle. But the resistance of the
friction is less than the strength with which the earth tugs at the
water, and therefore the wheel goes round and the water rushes down. The
force which really grinds the hard corn into flour it terrestrial
attraction! Gravitation of material substance towards material
substance, acting with an energy proportioned to the relative masses
and to the relative distances of the elements concerned.

Let us now suppose that the matter drawn towards the earth is not free
to move. Let us fancy, for instance, a drop of the running water all at
once stopped in its downward path by the attachment of a string from
above. The earth would then tug at that string in its effort to get the
drop of water, and would consequently stretch it to a certain extent.
The power that was before expended in causing the drop to move, would be
now employed in striving to tear asunder the substance of the string. A
heavy body hanging by a cord from a fixed point is then in this
predicament. It is drawn towards the earth, but is prevented from moving
to it. It consequently finds a position of rest in which it is placed as
near to the source of attraction as the suspending string allows; that
is, it hangs perpendicularly and immovably beneath it, stretching the
string by its tendency toward the ground.

If, however, the suspended body be raised up from its position of forced
repose by any interference that draws it to one side, the string being