I resolved, therefore, on making some experiments with my son Emile, and, in order to make my young assistant understand the nature of the exercise we were going to learn, I took a domino, the cinq-quatre for instance, and laid it before him. Instead of letting him count the points of the two numbers, I requested the boy to tell me the total at once. “Nine,” he said. Then I added another domino, the quarter-tray. “That makes sixteen,” he said, without any hesitation. I stopped the first lesson here; the next day we succeeded in counting at a single glance four dominoes, the day after six, and thus we at length were enabled to give instantaneously the product of a dozen dominoes. This result obtained, we applied ourselves to a far more difficult task, over which we spent a month. My son and I passed rapidly before a toy-shop, or any other displaying a variety of wares, and cast an attentive glance upon it. A few steps further on we drew paper and pencil from our pockets, and tried which could describe the greater number of objects seen in passing. I must own that my son reached a perfection far greater than mine, for he could often write down forty objects, while I could scarce reach thirty. Often feeling vexed at this defeat, I would return to the shop and verify his statement, but he rarely made a mistake. My male readers will certainly understand the possibility of this, but they will recognize the difficulty. As for my lady readers, I am convinced beforehand they will not be of the same opinion, for they daily perform far more astounding feats. Thus, for instance, I can safely assert that a lady seeing another pass at full speed in a carriage, will have had time to analyze her toilette from her bonnet to her shoes, and be able to describe not only the fashion and quality of the stuffs, but also say if the lace be real, or only machine made. I have known ladies do this. This natural, or acquired, faculty among ladies, but which my son and I had only gained by constant practice, was of great service in my performances, for while I was executing my tricks, I could see everything that passed around me, and thus prepare to foil any difficulties presented me. This exercise had given me, so to speak, the power of following two ideas simultaneously, and nothing is more favorable in conjuring than to be able to think at the same time both of what you are saying and of what you are doing. I eventually acquired such a knack in this, that I frequently invented new tricks while going through my performances. One day, even, I made a bet I would solve a problem in mechanics while taking my part in conversation. We were talking of the pleasure of a country life, and I calculated during this time the quantity of wheels and pinions, as well as the necessary cogs, to produce certain revolutions required, without once failing in my reply.