Modern Magic: A Practical Treatise on the Art of Conjuring

If you professedly use a borrowed florin, you must adroitly change it for your own, under pretence of showing how to spin it, or the like. You should not allow your audience to imagine that you are guided by the sound of the coin, as, if once they have the clue, they will easily learn to distinguish the two sounds. They are not, however, likely to discover the secret of the notch, and if any one professes to have found out the trick, you may, by again substituting an unprepared florin, safely challenge him to perform it. ODD OR EVEN, OR THE MYSTERIOUS ADDITION.--This is a trick of almost childish simplicity, depending upon an elementary arithmetical principle. We have, however, known it to occasion great perplexity, even to more than ordinarily acute persons. You take a handful of coins or counters, 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.