When he has made his choice, desire him to lay on the table his three cards separately, and to put upon each parcel as many cards as wanting to make up 15 points; that is to say, if the first card should be a nine, there must be added six cards over; if the second a ten, five cards; and if the third a knave, five cards likewise; this will make nineteen cards employed; consequently there will remain thirteen, which you are to ask for; and pretending to examine them, you must count them in order to be certain of the number that is left; then in your mind add sixteen to the remaining number, and you will have twenty-nine, number of the points that the three chosen cards under the parcels contain. CHAP. XVIII. _Sympathetic Inks._ These kinds of inks are very curious, and may serve for a great number of physical recreations, very surprising to such as are not acquainted with the manner of preparing them. One kind, very easy, is made by taking an ounce of common aqua fortis, which you are to mix with three ounces of common water; you will use this mixture to write on paper that is strong and very stiff: this writing becomes totally invisible in drying; and in order to make it reappear, you need only wet the paper; and when it dries the writing disappears again. This effect may be repeated two or three times. This process is the easiest to be done, as the necessary ingredients are almost always at hand. Many other things furnish the means of making sympathetic ink, such as cobalt, bismuth, lime, &c. &c. but they require chemical and difficult preparations to be efficient. The easiest to be obtained are mentioned before; as the mixture of aqua fortis and common water; and those that may be formed by dissolutions of salt and acids, such as lemon or onion juice: in order to render them visible, you need only approach them to the fire: the cold air produces on them the contrary effect. CHAP. XIX. _To make an addition before the Figures are set, by knowing only how many Figures are in each Row; as likewise how many Rows compose the whole; and then adding yourself some Figures equal to those that had been set._ Suppose the person had set five rows of figures, each row containing five figures. Say in your mind, as you are making the addition beforehand, 9 times 5 make 45; set down 5 and carry 4: repeat the same thing for each of the five figures, as if they all counted 9; therefore for the second, say again, 9 times 5 make 45, and 4 carried over make 49; set down 9 and carry 4: in the same manner for the third, say 9 times 5 are 45, and 4 carried over are 49; set down 9 and carry 4: for the fourth do the same; and set down 9 and carry 4: for the fifth repeat the same, by setting down 9 and carrying 4.