pointed end is touched, on the pivot described in the centre of the circle, it will stop on one of the four pips, against which you have placed the pin in the frame; then take that needle off, and placing the other, it will stop at the opposite point.

Having matters thus arranged, desire a person to draw a card from a piquet pack, offering that card against which you have placed the pin of the dial, which you may easily do, by having a card a little longer than the rest. If he should not draw it the first time, as he probably may not, you must make some excuse for shuffling them again; such as letting the cards fall, as if by accident, or some other manœuvre, till he fixes on the card. You then tell him to keep it close, and not let it be seen. Then give him one of the two needles, and desire him to place it on the pivot, and turn it round, when it will stop at the colour of the card he chose; then taking that needle off, and exchanging it unperceived for the other, give it to a second person, telling him to do the same, and it will stop at the name of the identical card the first person

chose.

The Communicative Crown.

Take a crown piece, and bore a hole in the side of it; in which place a piece of wire, or a large needle well polished, and strongly touched with a magnet.Then close the hole with a small piece of pewter, that it may not be perceived. Now the needle in the magnetic perspective before described, when it is brought near to this piece of money, will fix itself in a direction correspondent to the wire or needle in that piece.

Desire any person to lend you a crown piece, which you dexterously change for one that you have prepared as above. Then give the latter piece to another person, and leave him at liberty either to put it privately in a snuff-box or not. He is then to place the box on a table, and you are to tell him, by means of your glass, if the crown is, or is not in the box, Then bringing your perspective close to the box, you will

know, by the motion of the needle, whether it be there or not; for as the needle in the perspective will always keep to the north of itself, if you do not perceive it has any motion, you conclude the crown is not in the box. It may happen, however, that the wire in the crown' may be placed to the north, in which case you will be deceived. Therefore, to be sure of success, when you find the needle in the perspective remain stationary, you make some pretence to desire the person to move the box into another position, by which you will cer tainly know if the crown piece be there or not.

You must remember that the needle in the perspective must here be very sensible, as the wire in the crown cannot possibly have any great attractive force.

The Magnetic Table.

Under the top of a common table place a magnet that turns on a pivot, and fix a board under it, that nothing may appear. There may also be a drawer under the table, which you pull out to show that there is nothing concealed. At one end of the table there must be a pin that communicates with the magnet, and by which it may be placed in different positions: this pin must be so placed as not to be visible by the spectators. Strew some steel filings, or very small nails, over that part of the table where the magnet is. Then ask any one to lend you a knife, or a key, which will then attract part of the nails or filings, in the same manner as the iron attracts the needle, in the note to the twelfth aphorism. Then placing your hand, in a careless manner, on the pin at the end of the table, you alter the position of the magnet: and giving the key to any person you desire him to make the experiment, which he will then not be able to perform. You then give the key to another person, at the same time placing the magnet, by means of the pin, in the first position, when that person will immediately perform the experiment.

The Incomprehensible Card.

Insert in the middle of a card, and parallel to its two longest sides, part of a watch spring, as thin as possible, and strongly impregnated: let it be so concealed as not to afford the least suspicion. This card should be a little longer than the others of the pack in which it is placed.

Offer any one to draw a card out of the pack, and present the long card dexterously to his hand. You then give him all the cards, and leave him to replace that card in the pack or not. He is then to lay the pack on the table, and by applying your magnetic perspective, you will discover whether the card be there or

not.

If the person should not draw that card, you must be ready with some other experiment, to prevent suspicion of having failed in your design.

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213

PNEUMATICS.

DEFINITIONS.

1. THE atmosphere is that body of air which every where surrounds the earth.

2. The air pump is a machine contrived to produce a vacuum, by exhausting the air out of the vessel called a receiver.

3. The condenser is an instrument generally în form of a syringe, to force a greater quantity of air into any vessel than it naturally contains.

4. The anometer is an instrument that measures the strength of the wind.

5. The hygrometer is contrived to show the different degrees of moisture in the atmosphere at different times. 6. The thermometer measures the degrees of heat and cold of the air, and of other bodies.

7. The barometer shows the different weight of the air at different times.

APHORISMS.

1. The air is an elastic, ponderating, compressible and expansible fluid, that is sensible only to the touch. 2. The elasticity of the air is increased by heat and decreased by cold.

3. The weight of the air is so small as not to be perceived but in large quantities.

4. The rarefaction and condensation of the air, are indefinite.

5. Though air is greatly condensible by cold, it cannot be congealed.

6. Air is necessary to animal existence.

7. Adust air, that is, such as has passed through the fire or a heated tube, will not support animal life.

8. Air is contained in almost all bodies, and may be produced from them.

9. Sound is communicated by the air.

10. The atmosphere is of different densities at different heights, and is most dense near the earth.

11. The height of the atmosphere does not exceed 50 miles.

12. Wind is nothing but a current of air.

13. The velocity of the wind is from 1 to 60 miles in an hour.

To describe the numerous apparatus necessary for experimenting on air, among which the Air Pump, the Animometer, and Hygrometers, are the most conspicuously useful, would occupy more space than the limits of our small volume can allow. We shall therefore refer the reader to works professedly devoted to researches in Natural Philosophy, for descriptions and illustrations of these instruments, and proceed to enumerate a few amusing experiments in this branch of science.

The Bottle broke by Air.

Take a bottle that is square, not round or cylindrical; and if it be small, the glass must be thin, Put the mouth of this bottle over the hole in the place of the air-pump, and exhaust the air. By this means the bottle will be made to sustain the weight of the external air as long as it is able, but at last it will be suddenly burst into very small parts.

The same effect may be produced by the spring of the air, in the following manner. Seal the mouth of

a bottle so close that not the least air can come out, and place it in the receiver; then as the air is drawn off from its surface, the spring of the included air will act against the sides of the bottle, and will continually increase as the air in the receiver becomes more rarified till at last it burst the bottle in pieces.