s once been in
the mind. All mental activities, all knowledge which ever existed,
persists. We never wholly lose them, but they become faint and obscure.
One mental image effaces another. But those which have thus disappeared
may be recalled by an act of reminiscence. While it may sometimes be
impossible to recover one of them at the moment when wanted, by an act of
voluntary recollection, some association may bring it unexpectedly and
vividly before us. Memory plays us many strange tricks, both when we wake
and when we dream. It revives, by an involuntary process, an infinite
variety of past scenes, faces, events, ideas, emotions, passions,
conversations, and written or printed pages, all of which we may have
fancied had passed forever from our consciousness.

The aids to memory supposed to be furnished by the various mnemonic
systems may now be briefly considered. These methods of supplying the
defects of a naturally weak memory, or of strengthening a fairly good
one, are one and all artificial. This might not be a conclusive
objection to them, were they really effective and permanent helps,
enabling one who has learned them to recall with certainty ideas, names,
dates, and events which he is unable to recall by other means. Theory
apart, it is conceded that a system of memorizing which had proved widely
or generally successful in making a good memory out of a poor one, would
deserve much credit. But experience with these systems has as yet failed
to show, by the stern test of practical utility, that they can give
substantial (and still less permanent) aid in curing the defects of
memory. Most of the systems of mnemonics that have been invented are
constructed on the principle of locality, or of utilizing objects which
appeal to the sight. There is nothing new in these methods, for the
principle is as old as Simonides, who lived in the fifth century before
Christ, and who devised a system of memorizing by locality. One of the
most prevalent systems now taught is to select a number of rooms in a
house (in the mind's eye, of course) and divide the walls and the floors
of each room into nine equal parts or squares, three in a row. Then

"On the front wall--that opposite the entrance of the first
room--are the units, on the right-hand wall the tens, on the left
hand the twenties, on the fourth wall the thirties, and on the
floor the forties. Numbers 10, 20, 30, and 40, each find a place
on the ro