The secret lies in the use of a quarter of your own, on one face of which (say on the “tail” side) you have cut at the extreme edge a little notch, thereby causing a minute point or tooth of metal to project from that side of the coin. If a coin so prepared be spun on the table, and should chance to go down with the notched side upward, it will run down like an ordinary coin, with a long continuous “whirr,” the sound growing fainter and fainter till it finally ceases; but if it should run down with the notched side downward, the friction of the point against the table will reduce this final whirr to half its ordinary length, and the coin will finally go down with a sort of “flop.” The difference of sound is not sufficiently marked to attract the notice of the spectators, but is perfectly distinguishable by an attentive ear. If, therefore, you have notched the coin on the “tail” side, and it runs down slowly, you will cry “tail;” if quickly, “head.”
If you professedly use a borrowed coin, you must adroitly change it for your own, under pretence of showing how to spin it, or the like.
Odd or Even; or, the Mysterious Addition.
You take a handful of coins, and invite another person to do the same, and to ascertain privately whether the number he has taken is odd or even. You request the company to observe that you have not asked him a single question, but that you are able, notwithstanding, to divine and counteract his most secret intentions, and that you will, in proof of this, yourself take a number of coins and add them to those he has taken, when, if his number was odd, the total shall be even; if his number was even, the total shall be odd. Requesting him to drop the coins he holds into a hat, held on high by one of the company, you drop in a certain number on your own account. He is now asked whether his number was odd or even; and, the coins being counted, the total number proves to be, as you stated, exactly the reverse. The experiment is tried again and again, with different numbers, but the result is the same.
The secret lies in the simple arithmetical fact, that if you add an odd number to an even number, the result will be odd; if you add an odd number to an odd number, the result will be even. You have only to take care, therefore, that the number you yourself add, whether large or small, shall always be odd.
To Rub One Dime Into Three.
This is a simple little parlor trick, but will sometimes occasion a good deal of wonderment. Procure three dimes of the same issue, and privately stick two of them with wax to the under side of a table, at about half an inch from the edge, and eight or ten inches apart. Announce to the company that you are about to teach them how to make money. Turn up your sleeves, and take the third dime in your right hand, drawing particular attention to its date and general appearance, and indirectly to the fact that you have no other coin concealed in your hands. Turning back the table-cover, rub the dime with the ball of the thumb backward and forward on the edge of the table. In this position your fingers will naturally be below the edge. After rubbing for a few seconds, say, “It is nearly done, for the dime is getting hot;” and, after rubbing a moment or two longer with increased rapidity, draw the hand away sharply, bringing away with it one of the concealed dimes, which you exhibit as produced by the friction. Leaving the waxed dime on the table, and again showing that you have but one coin in your hands, repeat the operation with the remaining dime.
The Capital Q.
Take a number of coins, say from five-and-twenty to thirty, and arrange them in the form of the letter Q, making the “tail” consist of some six or seven coins. Then invite some person (during your absence from the room) to count any number he pleases, beginning at the tip of the tail and traveling up the left side of the circle, touching each coin as he does so; then to work back again from the coin at which he stops (calling such coin one), this time, however, not returning down the tail, but continuing round the opposite side of the circle to the same number. During this process you retire, but on your return you indicate with unerring accuracy the coin at which he left off. In order to show (apparently) that the trick does not depend on any arithmetical principle, you reconstruct the Q, or invite the spectators to do so, with a different number of coins, but the result is the same.
The solution lies in the fact that the coin at which the spectator ends will necessarily be at the same distance from the root of the tail as there are coins in the tail itself. Thus, suppose that there are five coins in the tail, and that the spectator makes up his mind to count eleven. He commences from the tip of the tail, and counts up the left side of the circle. This brings him to the sixth coin beyond the tail. He then retrogrades, and calling that coin “one,” counts eleven in the opposite direction. This necessarily brings him to the fifth coin from the tail on the opposite side, being the length of the tail over and above those coins which are common to both processes. If he chooses ten, twelve, or any other number, he will still, in counting back again, end at the same point.