Modern Magic: A Practical Treatise on the Art of Conjuring

This capital little trick depends upon a simple arithmetical principle; but the secret is so well disguised that it is very rarely discovered. All that the performer has to do is to count in his own mind the taps he gives, calling the first “one,” the second “two,” and so on. The first seven taps may be given upon any figures of the dial indifferently; indeed, they might equally well be given on the back of the watch, or anywhere else, without prejudice to the ultimate result. But the eighth tap must be given invariably on the figure “twelve” of the dial, and thenceforward the pencil must travel through the figures seriatim, but in reverse order, “eleven,” “ten,” “nine,” and so on. By following this process it will be found that at the tap which, counting from the number the spectator thought of, will make twenty, the pencil will have travelled back to that very number. A few illustrations will make this clear. Let us suppose, for instance, that the hour the spectator thought of was twelve. In this case he will count the first tap of the pencil as thirteen, the second as fourteen, and so on. The eighth tap in this case will complete the twenty, and the reader will remember that, according to the directions we have given, he is at the eighth tap always to let his pencil fall on the number twelve; so that when the spectator, having mentally reached the number twenty, cries, “Stop,” the pencil will be pointing to that number. Suppose, again, the number thought of was “eleven.” Here the first tap will be counted as “twelve,” and the ninth (at which, according to the rule, the pencil will be resting on eleven) will make the twenty. Taking again the smallest number that can be thought of, “one,” here the first tap will be counted by the spectator as “two,” and the eighth, at which the pencil reaches twelve, will count as “nine.” Henceforth the pencil will travel regularly backward round the dial, and at the nineteenth tap (completing the twenty, as counted by the spectator) will have just reached the figure “one.”