haust the ether from a given space, as it can exhaust the air,
the one could be proved as readily as the other. The difficulty of doing
that lies in the fact that matter in the etheric condition freely
inter-penetrates matter in all states below it, so that there is as yet no
means within the knowledge of our physicists by which any given body of
ether can be isolated from the rest. Practical Occultism, however, teaches
how this can be done, and thus the tremendous force of etheric pressure can
be brought into play. Thirdly, there is a vast store of potential energy
which has become dormant in matter during the involution of the subtle into
the gross, and by changing the condition of the matter some of this may be
liberated and utilized, somewhat as latent energy in the form of heat may
be liberated by a change in the condition of visible matter. Fourthly, many
striking results, both great and small, may be produced by an extension of
a principle which may be described as that of sympathetic vibration.
Illustrations taken from the physical plane seem generally to misrepresent
rather than elucidate astral phenomena, because they can never be more than
partially applicable; but the recollection of two simple facts of ordinary
life may help to make this important branch of our subject clearer, if we
are careful not to push the analogy further than it will hold good. It is
well known that if one of the wires of a harp be made to vibrate
vigorously, its movement will call forth sympathetic vibrations in the
corresponding strings of any number of harps placed round it, if they are
tuned to exactly the same pitch. It is also well known that when a large
body of soldiers crosses a suspension bridge it is necessary for them to
break step, since the perfect regularity of their ordinary march would set
up a vibration in the bridge which would be intensified by every step they
took, until the point of resistance of the iron was passed, when the whole
structure would fly to pieces. With these two analogies in our minds (never
forgetting that they are only partial ones) it may seem more comprehensible
that one who knows exactly at what rate to start his vibrations--knows, so
to speak, the keynote of the class of matter he wishes to affect--should be
able by sounding that keynote to call forth an immense number of
sympathetic vibrations. When this is done on the physical plane no
additional energy is developed; but on the astral plane the