THE MAGIC TOP—THE GYROSCOPE AND SCIENTIFIC GAMES

We will not do our readers the injustice to suppose that they are not familiar with the ordinary top,—the delight of all school-boys and young people,—of which, therefore, we forbear giving any description; but we now desire to give some details of the construction of the wonderful magic top. It is composed of a large disc, with an axis turning on two pivots connected with a circle of iron. When in repose, this plaything exhibits nothing of a remarkable character; it is completely inert, obeying, like all other bodies, the laws of gravity. But when we come to give the disc a movement of rapid rotation, this inert instrument seems to assume a vitality of its own if we attempt to move it; it resists, and seems to thrust back the hand, and executes movements even in a contrary direction. Besides this, it appears to be freed, in a certain measure, from the laws of gravity; if we place it on its pivot, instead of falling, as it would when the disc is motionless, it preserves the upright or inclined position in which we place it, the upper extremity of the axis slowly describing a horizontal circle round the fulcrum of the other extremity.

Few persons are sufficiently familiar with the theory of mechanics to understand these phenomena, and it often happens that such a top purchased to amuse a child becomes an object of wonder and interest to his seniors. We do not pretend here to explain mathematically the reason of the facts before us, but the mechanical principle on which this top is constructed is of such great scientific importance, that we will, in a few words, explain it to our readers. It is sufficient to have a little knowledge of mechanics to be aware that a body in motion, subjected to the action of a force tending to give it a directly contrary motion, will follow a movement in a third direction, which is termed the resultant of the two others; and this resultant approaches nearer to one of the original directions, in proportion as the corresponding movement is more rapid in relation to the other. If, for example, you strike a billiard ball, which is rolling past you, in such a manner that you drive it regularly along in the same direction, it appears only to obey a part of the given impulsion, and continues its course in an oblique direction, the speed with which it commenced rolling combining with the impulsion to produce a resultant movement. If it is rolling very quickly, and you strike it gently, it will scarcely turn out of its course. If, on the contrary, it is moving slowly, and receives a violent shock, it will run off almost exactly in the direction in which it has been struck.

Now that which occurs in this example of a body tending to two movements at the same time, is also produced when it is a question of movements of rotation, so that if a force acts upon a body in rotation in such a manner as to give it a movement of the same kind round another axis, a third movement will be originated round a third axis, the direction of which will be nearest to that in which the rotation is most rapid. Let us apply this very simple principle to our top, and we shall see that magic has nothing whatever to do with these movements, which at first glance appears so extraordinary. Having set it in motion, we rest it on its pivot, its axis in a horizontal position; we then find that we have two movements before us; first, that which we gave the top ourselves, and secondly, the movement of rotation which occurs round a second axis equally horizontal passing through the fulcrum and perpendicular line to the first. A movement of rotation therefore originates round a third axis placed between the two first, but whilst the real axis of the top, obeying this resultant movement, takes up its new position, the law of gravity continuing to act, displaces and moves it a little further, so that in endeavouring to reach its centre of gravity, it turns round its fulcrum (fig. 871). From this explanation, it will be easily seen that the more rapid the movement given to the top,—that due to gravity remaining constant,—the nearer will be the axis of the resultant movement to its real axis, and consequently the slower will be the movement of rotation of the whole round the pivot. Thus this apparently incomprehensible phenomenon is easily explained by gravity, vertical force producing a movement of rotation in a horizontal plane. One can also explain by analogous reasoning, and calculation of passive resistance, why the axis of the top gradually inclines in proportion as the speed of the latter diminishes, and the speed of rotation round the fulcrum increases; why it falls immediately if an obstacle is opposed to the latter movement, and finally, why it produces on the hand which holds it, movements which astonish persons so intensely who behold it for the first time.

The principle we have just described is often enunciated, by saying that every body in rapid rotation rests in its plane, and can only be driven out by a considerable force; this, however, is a defective formula. The principle should be stated in the following manner. A body in rapid rotation tends to remain in its plane; that is, its axis rests parallel with itself, and instead of obeying any force tending to divert its direction, it produces in consequence of the combination of two simultaneous movements, a displacement of the axis, generally much feebler and of a different kind from that which this force exercises on the same body in repose. One of the most charming applications of this theory is due to M. Foucault. The Gyroscope, which bears his name, is a heavy disc, the axis of which is supported by a “Cardan” balance, so that, whatever is the position of the contrivance, it is possible to preserve it in a constant direction. Therefore if the disc is, by means of special mechanism, put in rapid rotation, we may give it all kinds of possible displacement without changing the plane in which the gyroscope moves. Supposing then that its connection with the suspension is fixed in a relatively immovable manner, but attracted by a movement towards the ground, the plane of rotation of the disc will not entirely participate in this movement. It is true, it will be carried into the movement of general removal, but it will remain constantly parallel with itself, and only appears displaced in comparison with the surrounding objects, which obey more completely than itself the movement of the globe’s rotation round its poles. Thus can we demonstrate the movement of our planet. In virtue of the same principle, we see every day passing before our eyes a crowd of phenomena with which we are so familiar that they do not excite our attention. Thus it is because the hoop tends to remain in its plane of rotation that it rolls on without falling or deviating, and for the same reason that tops rotate vertically on their points, or when they are running down, describe a series of concentric circles; and for the same reason again, a juggler is able so easily to hold on the point of a stick a plate which he puts in rapid rotation, etc. It is also owing to this property of rotating bodies that we have been enabled to make use of cylindrical or conical projectiles in artillery. The coiled riflings of the cannon causing the projectiles to turn round very rapidly, their axis preserves an invariable direction during their whole course, until they finally strike the object at which they are aimed. Without this rotation they would pirouette in an irregular manner in the air, and besides any precision in firing being impossible, the resistance of the air would diminish their range to an enormous extent.

The Gyroscope, an instrument now familiar to most scientific persons, is still a problem of which the solution has not yet been found. It may be called the paradox of mechanics, for although it depends on gravitation, it appears to be entirely indifferent to it.

An American scientist has applied electricity to the gyroscope, so as to make its movements as continuous as possible, and to enable us to study it more at leisure and with better results. The gyroscope is mounted on a pedestal which tapers to a point, and supports the instrument. The bar of the gyroscope on which the electro-magnets are fixed rests upon the top of the pedestal. One of the extremities of the bobbin is fixed to the cavity, when the bar and support join, the other extremity communicates with the bar which joins the nuts of the magnets.

An insulator of hardened caoutchouc is so placed that it just touches the axis of the wheel twice in every revolution of that wheel. Its plane of rotation is at right angles to the magnets, and carries an armature of soft iron which turns very close to the magnet without touching it. The armature is put en rapport with the surface of contact of the cylinder, so that when the armature approaches it is attracted; but immediately afterwards, as it reaches the opposite side, the current is interrupted, and the impulse acquired is sufficient to move the wheel to the spot where the armature can again come under the influence of the magnet.

The magnets, the wheel, and all the parts of the instrument together can move around in any direction. When two or four Bunsen cells are put in connection with the gyroscope, the wheel turns with tremendous rapidity, and by permitting the magnets to work (an operation which requires some little dexterity), the wheel not only sustains itself, but also the magnets and the other subjects which are between it and the extremity of the pedestal—in opposition to the laws of gravitation. The wheel, besides turning rapidly around its axis; revolves slowly around the point of the column in the same direction taken by the lower part of the wheel.

When attaching the arms and counter-poise of the machine, so that the wheel and the magnets may balance themselves exactly on the pointed pedestal, the machine remains stationary. But if we give any preponderance to the wheel and magnets the rotation of the apparatus is in a direction opposite to that which would result from turning the upper part of the wheel.

The gyroscope illustrates the persistence with which a body that submits to rotation maintains itself in the plane of its rotation, notwithstanding the force of gravitation. It also shows the result of the combined action of two forces tending to produce rotation around two separate axes, which are, however, situated in the same plane.

The rotation of the wheel round its axis is produced in the present case by the electro-magnet; and the tendency of the wheel to fall, or to turn in a vertical plane parallel to its axis, results in the rotation of the entire instrument upon a new axis, which coincides with the pointed pedestal.

The American Money-Box.

During a recent visit to London, as I was one day walking in the Crystal Palace, my attention was attracted by a curious money-box, surmounted by a pictorial representation of one of the London streets. The carriages, horses, and pedestrians were represented by figures cut out of cardboard, arranged in a groove. A large placard bore this inscription: “Notice to visitors: Throw a penny in the money-box; and the figures will perform.”

I at once responded to this invitation, and immediately beheld the little tableaux become moving and life-like; the cabs rolled along, and the passers-by walked up and down the street. A number of visitors followed my example, and there is no doubt the money-box was full at the end of the day. This ingenious contrivance for obtaining money in so easy a manner, and without having recourse to a “show-man,” appeared to me worthy of investigation and description.

The Scientific American (New York) has recently given an explanation of this curious contrivance, and we will here quote what has been published on the subject.

“Among the inventions intended to obtain contributions of money from the visitors at the Philadelphia Exhibition,” says the American writer, “we will describe the singular money-boxes placed in the salons of the principal hotels and the galleries of the exhibition, etc. These contrivances all consisted of a case or box, with a glass front, through which can be seen a landscape in miniature, with trees, houses, figures, etc., all cut out of cardboard, and painted with great nicety. On the box was a label requesting the visitor to drop a coin into it and await the result of the contribution. When the penny has fallen in it puts in motion some hidden machinery, and then we see the people in the miniature landscape all in motion, riding or walking or hunting, as the case may be.”

Another box is even more successful, for it places in the hands of the contributor a photograph of some celebrated person. But to obtain the photograph we must contribute six pennies. The carte will not come out if we do not put in the proper coins, and the apparatus is perfectly fair and honest.

The illustration, fig. 872, shows the apparatus, which is very simple. On the left the ordinary box is seen, on the right there is a longitudinal section of it.

At the top of the lower portion, where the money is received in, is a hollow support, A, which sustains the box in which the photographs are placed upon an inclined plane, and resting against the glass. The pieces of money, in falling, strike the extremity of a vertical balance, which immediately turns a toothed wheel, C. This wheel has as many teeth as there are pieces of money necessary to purchase the photograph or carte de visite. Upon the escapement wheel is a ratchet arrangement, D, the shaft being moved by a cord rolled around it and attached to a spring, E. A bolt, F, moved by a spring, is kept constantly pressed against the “snail,” D.

Thus at each revolution, as the parts of the machinery are animated by the same movement, the bolt is withdrawn sufficiently to permit a carte to fall, and then the card next following will be ready resting upon the bolt. The photographs being placed upon an inclined plane, are pushed forward by a movable frame, G, which has a roller at the base. So as one card falls out another is immediately replaced close to the glass.

We have remarked that the wheel has six teeth, so that as one piece of money dropped in moves it one-sixth of its revolution, six pieces will be necessary to produce the card. Of course wheels can be made with one or more teeth, and the payment may be varied for valuable objects at the desire of the possessor.

The invention is not only a plaything. It can be made useful in the distribution of pamphlets, or newspapers, which can be introduced into the box folded uniformly. They can also be used in omnibuses or tram-cars, and tickets may be given by the machine on payment of the proper sum of money.

We will close this chapter with an illustration of a spiral bottle, which can be done in the manner now to be described, so that the bottle will actually become a glass spring.

Take a mixture of 180 grammes of lampblack, 60 grammes of gum arabic, 23 grammes of adraganth, and 23 grammes of benzoin. Make these ingredients into a paste by the addition of water, and fashion a pencil of the charcoal thus obtained. This pencil, when heated, will cut the glass wherever it is applied.

The process is commenced by scraping the bottle with a file and following the instrument with the red-hot pencil. Wherever the hot pencil is applied, the glass will be cut as shown in the illustration herewith. It will be necessary to blow upon the heated pencil to maintain the incandescence as long as possible. The bottle as cut and representations of the instruments are given in the cut (fig. 873).