he has as clearly seen whence those clattering wheels derive their many horse-power!  If we were to ask him to tell us how they acquired their rolling strength, he would most probably answer—­from the current of the stream.  This reply would amount to nothing in the matter of explanation; the force of the current is as much a borrowed attribute as the force of the wheelwork.  The running water is no more an independent and living agent than is the machinery which it turns.  Beyond both is the one grand determining influence—­the attractive energy inherent in the substance of the vast earth.  This it is which makes the water run; this it is which enables the running water to move the wheelwork inserted into its channel.  As the magnet draws to itself the fragment of steel, the earth draws to itself all ponderable matter; and whenever ponderable matter is free to move, it rushes as far as it can go towards the centre of the earth’s 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.