through with a penknife, and the space cut out between each spiral
should be three or four times as wide as the spirals themselves. You
must observe to cut them so that the pyramid or globe may appear to
turn on its axis. The columns that are represented in pieces of
architecture, or in jets of fire, must be cut in the same manner, if
they are to be represented as turning on their axis.
In like manner may be exhibited a great variety of ornaments, ciphers,
and medallions, which, when properly coloured, cannot fail of
producing the most pleasing effect. There should not be a very great
diversity of colours, as they would not produce the most agreeable
appearance.
When these pieces are drawn on a large scale, the architecture or
ornaments may be shaded; and, to represent different shades, pieces of
coloured paper must be pasted over each other, which will produce an
effect that would not be expected from transparent paintings. Five or
six pieces of paper pasted over each other will be sufficient to
represent the strongest shades.
To give these pieces the different motions they require, you must
first consider the nature of each piece; if, for example, you have cut
out the figure of the sun, or of a star, you must construct a wire
wheel of the same diameter with these pieces; over this wheel you
paste a very thin paper, on which is drawn, with black ink, the spiral
figure. The wheel thus prepared, is to be placed behind the sun or
star, in such a manner that its axis may be exactly opposite the
centre of either of these figures. This wheel may be turned by any
method you think proper.
Now, the wheel being placed directly behind the sun, for example, and
very near to it, is to be turned regularly round, and strongly
illuminated by candles placed behind it. The lines that form the
spiral will then appear, through the spaces cut out from the sun, to
proceed from its centre to its circumference, and will resemble sparks
of fire that incessantly succeed each other. The same effect will be
produced by the star or by any other figure where the fire is not to
appear as proceeding from the circumference of the centre.
These two pieces, as well as those that follow, may be of any size,
provided you observe the proportion between the parts of the figure
and the spiral, which must be wider in larger figures than in small.
If the sun, for example, have from six to twelve inches diameter, the
width of the strokes that form the spiral need not be more than
one-twentieth part of an inch, and the spaces between them, that form
transparent parts, about two-tenths of an inch. If the sun be two feet
diameter, the strokes should be one-eighth of an inch, and the space
between, one quarter of an inch; and if the figure be six feet
diameter, the strokes should be one quarter of an inch and the spaces
five-twelfths of an inch. These pieces have a pleasing effect, when
represented of a small size, but the deception is more striking when
they are of large dimensions.
It will be proper to place those pieces, when of a small size, in a
box quite closed on every side, that none of the light may be diffused
in the chamber: for which purpose it will be convenient to have a tin
door behind the box, to which the candlesticks may be soldered, and
the candles more easily lighted.
The several figures cut out should be placed in frames, that they may
be put, alternately, in a groove in the forepart of the box; or there
may be two grooves, that the second piece may be put in before the
first is taken out.
The wheel must be carefully concealed from the eye of the spectator.
Where there is an opportunity of representing these artificial fires
by a hole in the partition, they will doubtless have a much more
striking effect, as the spectator cannot then conjecture by what means
they are produced.
It is easy to conceive that by extending this method, wheels may be
constructed with three or four spirals, to which may be given
different directions. It is manifest also that, on the same principle,
a great variety of transparent figures may be contrived, and which may
be all placed before the spiral lines.
_To represent Cascades of Fire._
In cutting out cascades, you must take care to preserve a natural
inequality in the parts cut out; for if, to save time, you should make
all the holes with the same pointed tool, the uniformity of the parts
will not fail to produce a disagreeable effect. As these cascades are
very pleasing when well executed, so they are highly disgusting when
imperfect. These are the most difficult pieces to cut out.
To produce the apparent motion of these cascades, instead of drawing a
spiral, you must have a slip of strong paper, of such length as you
judge convenient. In this paper there must be a greater number of
holes near each other, and made with pointed tools of different
dimensions.
At each end of the paper, a part of the same size with the cascade
must be left uncut; and towards those parts the holes must be made at
a greater distance from each other.
When the cascade that is cut out is placed before the scroll of paper
just mentioned, and it is entirely wound upon the roller, the part of
the paper that is then between being quite opaque, no part of the
cascade will be visible; but as the winch is gently turned, and
regularly round, the transparent part of the paper will give to the
cascade the appearance of fire that descends in the same direction;
and the illusion will be so strong, that the spectators will think
they see a cascade of fire; especially if the figure be judiciously
cut out.
_The Oracular Mirror._
Provide a round mirror of about three inches in diameter and whose
frame is an inch wide. Line the under part of the frame, in which
holes are to be cut, with very thin glass; behind this glass let a
mirror of about two inches diameter be placed, which is to be
moveable, so that by inclining the frame to either side, part of the
mirror will be visible behind the glass on that side.
Then take Spanish chalk, or cypress vitriol, of which you make a
pencil, and with this you may write on a glass, and rub it off with a
cloth, and by breathing on the glass, the writing will appear and
disappear several times. With this pencil write on one side of the
mirror, before it is put in the frame, the word _yes_, and on the
other side, _no_; and wipe them off with a cloth.
You propose to a person to ask any question of this mirror that can be
answered by the words _yes_ or _no_. Then turning the glass to one
side, and putting your mouth close to it, as if to repeat the question
softly, you breathe on it, and the word yes or no will immediately
appear. This mirror will serve for many other agreeable amusements.
_The Hour of the Day or Night told by a suspended Shilling._
However improbable the following experiment may appear, it has been
proved by repeated trials:
Sling a shilling or sixpence at the end of a piece of thread by means
of a loop. Then resting your elbow on a table, hold the other end of
the thread betwixt your fore-finger and thumb, observing to let it
pass across the ball of the thumb, and thus suspend the shilling into
an empty goblet. Observe, your hand must be perfectly steady; and if
you find it difficult to keep it in an immoveable posture, it is
useless to attempt the experiment. Premising, however, that the
shilling is properly suspended, you will observe, that when it has
recovered its equilibrium, it will for a moment be stationary: it will
then of its own accord, and without the least agency from the person
holding it, assume the action of a pendulum, vibrating from side to
side of the glass, and, after a few seconds, will strike the hour
nearest to the time of day; for instance, if the time be twenty-five
minutes past six, it will strike six; if thirty-five minutes past six,
it will strike seven; and so on of any other hour.
It is necessary to observe, that the thread should lie over the pulse
of the thumb, and this may in some measure account for the _vibration_
of the shilling; but to what cause its striking the precise hour is to
be traced, remains unexplained; for it is no less astonishing than
true, that when it has struck the proper number, its vibration ceases,
it acquires a kind of rotatory motion, and at last becomes stationary,
as before.
_Of Lightning, and the best Method of guarding against its mischievous
Effects._
Experiments made in electricity first gave philosophers a suspicion,
that the matter of lightning was the same with the electric matter.
Experiments afterwards made on lightning obtained from the clouds by
pointed rods, received into bottles, and subjected to every trial,
have since proved this suspicion to be perfectly well founded; and
that, whatever properties we find in electricity, are also the
properties of lightning.
This matter of lightning, or of electricity, is an extreme subtle
fluid, penetrating other bodies, and subsisting in them, equally
diffused.
When, by any operation of art or nature, there happens to be a greater
proportion of this fluid in one body than in another, the body which
has most will communicate to that which has least, till the proportion
becomes equal, provided the distance between them be not too great;
or, if it be too great, till there be proper conductors to convey it
from one to the other.
If the communication be through the air, without any conductor, a
bright light is seen between the bodies, and a sound is heard. In
small experiments, we call this light and sound the electric spark and
snap; but in the great operations of nature, the light is what we call
_lightning_, and the sound (produced at the same time, though
generally arriving later at our ears than the light does in our eyes)
is, with its echoes, called _thunder_.
If the communication of this fluid be by a conductor, it may be
without either light or sound, the subtle fluid passing in the
substance of the conductor.
If the conductor be good, and of sufficient bigness, the fluid passes
through it without hurting it. If otherwise, it is damaged or
destroyed.
All metals, and water, are good conductors. Other bodies may become
conductors by having some quantity of water in them, as wood and other
materials used in building, but not having much water in them, are not
good conductors, and therefore are often damaged in the operation.
Glass, wax, silk, wool, hair, feathers, and even wood perfectly dry,
are non-conductors: that is, they resist instead of facilitating the
passage of this subtle fluid.
When this fluid has an opportunity of passing through two conductors,
one good and sufficient, as of metal, the other not so good, it passes
in the best, and will follow in any direction.
The distance at which a body charged with this fluid will discharge
itself suddenly, striking through the air into another body that is
not charged, or not so highly charged, is different according to the
quantity of the fluid, the dimensions and form of the bodies
themselves, and the state of the air between them. This distance,
whatever it happens to be between any two bodies, is called their
striking _distance_, as, till they come within that distance of each
other, no stroke will be made.
The clouds have often more of this fluid in proportion than the earth:
in which case, as soon as they come near enough, (that is, within the
striking distance,) or meet with a conductor, the fluid quits them and
strikes into the earth. A cloud fully charged with this fluid, if so
high as to be beyond the striking distance from the earth, passes
quietly without making noise or giving light, unless it meet with
other clouds that have less.
Tall trees and lofty buildings, as the towers and spires of churches,
become sometimes conductors between the clouds and the earth; but, not
being good ones, that is, not conveying the fluid freely, they are
often damaged.
Buildings that have their roofs covered with lead, or other metal, and
spouts of metal continued from the roof into the ground to carry off
the water, are never hurt by lightning, as, whenever it falls on such
a building, it passes in the metals and not in the walls.
When other buildings happen to be within the striking distance from
such clouds, the fluid passes in the walls, whether of wood, brick, or
stone, quitting the wall only when it can find better conductors near
them, as metal rods, bolts, and hinges of windows or doors, gilding on
wainscot, or frames of pictures, the silvering on the backs of
looking-glasses, the wires for bells, and the bodies of animals, so
containing watery fluids. And in passing through the house it follows
the direction of these conductors, taking as many in its way as can
assist in its passage, whether in a straight or crooked line, leaping
from one to the other, if not far distant from each other, only
rending the wall in the spaces where these partial good conductors are
too distant from each other.
An iron rod being placed on the outside of a building, from the
highest part continued down into the moist earth, in any direction,
straight or crooked, following the form of the roof or other parts of
the building, will receive the lightning at its upper end, attracting
it so as to prevent its striking any other part; and, affording it a
good conveyance into the earth, will prevent its damaging any part of
the building.
A small quantity of metal is found able to conduct a quantity of this
fluid. A wire no higher than a goose-quill has been known to conduct
(with safety to the building, as far as the wire was continued) a
quantity of lightning that did prodigious damage both above and below
it; and probably larger rods are not necessary, though it is common in
America to make them of half an inch, some three-quarters, or an inch,
diameter.
The rod may be fastened to the wall, chimney, &c., with staples of
iron. The lightning will not leave the rod (a good conductor) to pass
into the wall (a bad conductor) through those staples. It would
rather, if any were in the wall, pass out of it into the rod, to get
more readily by that conductor into the earth.
If the building be very large and extensive, two or more rods may be
placed in different parts, for greater security.
Small ragged parts of clouds, suspended in the air between the great
body of clouds and the earth, (like leaf gold in electrical
experiments,) often serve as partial conductors for the lightning,
which proceeds from one of them to another, and by their help comes
within the striking distance to the earth or a building. It therefore
strikes, through those conductors, a building that would otherwise be
out of the striking distance.
Long sharp points communicating with the earth, and presented to such
parts of clouds, drawing silently from them the fluid they are charged
with, they are then attracted to the cloud, and may leave the distance
so great as to be beyond the reach of striking.
It is therefore that we elevate the upper end of the rod, six or eight
feet above the highest part of the building, tapering it gradually to
a fine sharp point, which is gilt, to prevent its rusting.
Thus the pointed rod either presents a stroke from the cloud, or if a
stroke be made, conducts it to the earth, with safety to the building.
The lower end of the rod should enter the earth so deep as to come at
the moist part, perhaps two or three feet; and if bent when under the
surface, so as to go in a horizontal line six or eight feet from the
wall, and then bent again downwards three or four feet, it will
prevent damage to any of the stones of the foundation.
A person apprehensive of danger from lightning, happening during the
time of thunder to be in a house not so secured, will do well to avoid
sitting near the chimney, near a looking-glass, or any gilt pictures
or wainscot; the safest place is in the middle of the room, (so it be
not under a metal lustre suspended by a chain,) sitting in one chair
and laying the feet up in another. It is still safer to bring two or
three mattresses or beds into the middle of the room, and, folding
them up double, place the chair upon them; for they, not being so good
conductors as the walls, the lightning will not choose an interrupted
course through the air of the room and the bedding, when it can go
through a continued better conductor, the wall. But where it can be
had, a hammock or swinging-bed, suspended by silk cords equally
distant from the walls on every side, and from the ceiling and floor
above and below, affords the safest situation a person can have in any
room whatever; and what, indeed, may be deemed quite free from danger
of any stroke by lightning.
_The Leech, a Prognosticator of the Weather._
Confine a leech in a large phial, three parts filled with rain water,
regularly changed twice a week, and placed on a window frame, fronting
the north. In fair and frosty weather it lies motionless, and rolled
up in a spiral form, at the bottom of the glass: but prior to rain or
snow, it creeps up to the top, where if the rain will be heavy and of
some continuance, it remains a considerable time; if trifling, it
quickly descends. Should the rain or snow be accompanied with wind, it
darts about its habitation with amazing celerity, and seldom ceases
until it begins to blow hard. If a storm of thunder or lightning be
approaching, it is exceedingly agitated, and expresses its feelings in
violent convulsive starts, at the top of the glass. It is remarkable
that however fine and serene the weather may be, and not the least
indication to change, either from the sky, the barometer, or any other
cause whatsoever, yet, if the animal ever shift its position, or move
in a desultory manner, so certain will the coincident results occur,
within thirty-six hours, frequently within twenty-four, and sometimes
in twelve; though its motions chiefly depend on the fall and duration
of the wet, and the strength of the wind.
_The Awn of Barley an Hydrometer._
The awn of barley is furnished with stiff points, which, like the
teeth of a saw, are all turned towards the point of it; as this long
awn lies upon the ground, it extends itself in the moist air of night,
and pushes forward the barley-corn, which it adheres to in the day; it
shortens as it dries; and, as these points prevent it from receding,
it draws up its pointed end, and thus, creeping like a worm, will
travel many feet from the parent stem. That very ingenious mechanic
philosopher, Mr. Edgworth, once made on this principle a wooden
automaton: its back consisted of soft fir-wood, about an inch square,
and four feet long, made of pieces cut the cross-way in respect to the
fibres of the wood, and glued together; it had two feet before, and
two behind, which supported the back horizontally, but were placed
with their extremities, which were armed with sharp points of iron,
bending backwards. Hence, in moist weather, the back lengthened, and
the two foremost feet were pushed forwards; in dry weather the hinder
feet were drawn after, as the obliquity of the points of the feet
prevented it from receding.
_The Power of Water when reduced to Vapour by Heat._
Whatever force water may have while its parts remain together, is
nothing, if compared to the almost incredible power with which its
parts are endued, when they are reduced to vapour by heat. Those
steams which we see rising from the surface of boiling water, and
which to us appear feeble, yet, if properly conducted, acquire immense
force. In the same manner as gunpowder has but small effect, if
suffered to expand at large, so the steam issuing from water is
impotent, where it is permitted to evaporate into the air; but where
confined in a narrow compass, as, for instance, where it rises in an
iron tube shut up on every side, it there exerts all the wonders of
its strength. _Muschenbrook_ has proved by experiment, that the force
of gunpowder is feeble when compared to that of rising steam. A
hundred and forty pounds of gunpowder blew up a weight of thirty
thousand pounds: but, on the other hand, a hundred and forty pounds of
water, converted by heat into steam, lifted a weight of seventy-seven
thousand pounds; and would lift a much greater, if there were means of
giving the steam more heat with safety; for the hotter the steam the
greater is its force.
_Artificial Memory._
In travelling along a road, the sight of the more remarkable scenes we
meet with, frequently puts us in mind of the subjects we were thinking
or talking of when we last saw them. Such facts, which were perfectly
familiar, even to the vulgar, might very naturally suggest the
possibility of assisting the memory, by establishing a connexion
between the ideas we wish to remember, and certain sensible objects,
which have been found from experience to make a permanent impression
on the mind. It was said, that a person contrived a method of
committing to memory the sermons which he was accustomed to hear, by
fixing his attention, during the different heads of the discourse, on
different compartments of the roof of the church, in such a manner as,
that when he afterwards saw the roof, or remembered the order in which
its compartments were disposed, he recollected the method which the
preacher had observed in treating his subject. This contrivance was
perfectly analogous to the topical memory of the ancients; an art
which, whatever be the opinion we entertain of its use, is certainly
entitled, in a high degree, to the praise of ingenuity.
Suppose you fix in your memory the different apartments in some very
large building, and that you had accustomed yourself to think of these
apartments always in the same invariable order. Suppose further, that,
in preparing yourself for a public discourse, in which you had
occasion to treat of a great variety of particulars, you were anxious
to fix in your memory the order you proposed to observe in the
communication of your ideas. It is evident, that by a proper division
of your subject into heads, and by connecting each head with a
particular apartment, (which you could easily do, by conceiving
yourself to be sitting in the apartment while you were studying the
part of your discourse you mean to connect with it,) the habitual
order in which these apartments occurred to your thoughts, would
present to you in the proper arrangement, and without any effort on
your part, the ideas of which you were to treat. It is also obvious,
that very little practice would enable you to avail yourself of this
contrivance, without any embarrassment or distraction of your
attention.
_To procure Hydrogen Gas._
Provide a phial with a cork stopper, through which is thrust a piece
of tobacco-pipe. Into the phial put a few pieces of zinc, or small
iron nails; on this pour a mixture, of equal parts of sulphuric acid
(oil of vitriol) and water, previously mixed in a tea-cup, to prevent
accidents. Replace the cork stopper, with a piece of tobacco-pipe in
it; the hydrogen gas will then be liberated through the pipe into a
small steam. Apply the flame of a candle or taper to this steam, and
it will immediately take fire, and burn with a clear flame until all
the hydrogen in the phial be exhausted. In this experiment the zinc or
iron, by the action of the acid, becomes oxygenized, and is dissolved,
thus taking the oxygen from the sulphuric acid and water; the hydrogen
(the other constituent part of the water) is thereby liberated, and
ascends.
_To fill a Bladder with Hydrogen Gas._
Apply a bladder, previously wetted and compressed, in order to squeeze
out all the common air, to the piece of tobacco-pipe inserted in the
cork stopper of the phial, (as described in the experiment above.) The
bladder will thus be filled with hydrogen gas.
_Exploding Gas Bubbles._
Adapt the end of a common tobacco-pipe to a bladder filled with
hydrogen gas, and dip the bowl of the pipe into soap-suds, prepared as
if for blowing up soap bubbles; squeeze out small portions of gas from
the bladder into the soap-suds, and the bubbles will ascend into the
air with very great rapidity, until they are out of sight. If a
lighted taper or candle be applied to the bubbles as they ascend from
the bowl of the pipe, they will explode with a loud noise.
_Another Method._