This fact suggests, that a fulcrum is
not necessarily a palpable substance: it may be pliant or movable. For
instance, if we fasten the string of a kite to a ball, this ball,
which represents the fulcrum, being set in motion by the kite, becomes
a movable fulcrum: a child also, holding the string in his hand, runs
from right to left without impeding the motion of the kite, of which
motion he is the movable fulcrum. Absolute stability, therefore, is
not a necessary condition of a fulcrum; it is sufficient that there
be, between the resistant force and the motive force, a difference of
intensity in favour of the former. Thus, in water, the fulcrum, being
liquid, is necessarily pliant and movable; yet it is quite possible,
as every child knows, to obtain in this element purchase sufficient to
steer the largest ships.
'In the air, which is a gas, the fulcrum being gaseous, must also be
movable; but although the air, being the most elastic body with which
we are acquainted, is therefore the least apt to furnish a fulcrum,
yet, as compressed air is capable of bursting the strongest metallic
receptacles, splitting the solid rock, and rending the bosom of the
earth, it would seem that we have only to act upon the air through
pressure, in order to obtain the requisite purchase from which to
steer.
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