everal
letters which may be changed into others, without any appearance of
the alteration, as the _a_ into _d_, the _c_ into _a_, _e_, _d_, _g_,
_o_, or _q_; the _i_ into _b_, _d_, or _l_; the _l_ into _t_; the _o_
into _a_, _d_, _g_, or _q_; the _v_ into _y_, &c.

Take a parcel of cards, suppose twenty, and on one of them write with
juice of lemon or onion, or vitriol and water, the word law, (these
letters should not be joined;) and on the other, with the same ink,
the words _old woman_; then holding them to the fire, they both become
visible. Now, you will observe, that by altering the _a_ in the word
_law_ into _d_, and adding _o_ before the _l_, and _oman_ after the
_w_, it becomes _old woman_. Therefore you make those alterations with
the invisible ink, and let it remain so. On the rest of the cards you
write any words you think fit.

Present the cards in such manner to two persons, that one of them
shall draw the word _law_, and the other the words _old woman_. You
then tell the person who drew the word _law_, that it shall disappear,
and the words on the other card shall be written in its place; and,
that you may not change the cards, desire each of the parties to write
his name on his card. Then putting the cards together, and holding
them before the fire, as if to dry the names just written, the word
_law_ will presently change into _old woman_.

_The Enchanted Palace._

On the six-sided plane A B C D E F, Fig. 21, draw six semi-diameters;
and on each of these place perpendicularly two plane mirrors, which
must join exactly at the centre, and which, placed back to back, must
be as thin as possible. Decorate the exterior boundary of this piece,
(which is at the extremity of the angles of the hexagon,) with six
columns, that at the same time serve to support the mirrors by grooves
formed on their inner sides. Add to these columns their entablatures,
and cover the edifice in whatever manner you please. In each one of
these six triangular spaces, contained between two mirrors, place
little figures of pasteboard, in relief, representing such subjects,
as, when seen in an hexagonal form, will produce an agreeable effect.
To these add small figures of enamel, and take particular care to
conceal by some object that has no relation to the subject, the place
where the mirrors join, which, as before observed, all meet in the
common centre.

[Illustration: Fig. 21.]

When you look into any one of the six opening