A CERTAIN NUMBER OF CARDS BEING SHOWN TO A PERSON, TO GUESS THAT WHICH HE HAS THOUGHT OF.
To perform this trick, the number of the cards must be divisible by 3; and to do it with more convenience, the number must be odd.
The first condition, at least, being supposed, the cards must be disposed in three heaps, with their faces turned upwards. Having then asked the person in which heap is the card thought of, place the heaps one above the other, in such a manner that the one containing the card thought of may be in the middle. Arrange the cards again in three heaps, and having asked in which of them is the card thought of, repeat the operation as before. Arrange them a third time in three heaps, and having once more asked the same question, form them all into one heap, that containing the card thought of being in the middle. The card thought of must then necessarily be the middle one; that is to say, if 15 cards have been employed, it will be the eighth from the top; if 21, the eleventh; if 27, the fourteenth; and so on. When the number of the cards is 24, it will be the twelfth, &c.