The method of using the bow was this: The slate was shaken violently once or twice, so as to distribute the black sand equally over the white surface, and then the bow was drawn perpendicularly down the edge of the slate, very rapidly, so as to produce a quick whistling sound. The effect produced upon the grains of sand was truly wonderful to the uninitiated in the laws of acoustics. They arranged themselves into peculiar figures, sometimes in the form of a semicircle, sometimes into that of a spiral, sometimes into a perfect circle, or a cone, or a rhomboid, or an oval, dependent entirely upon two things: first, the place where the slate was held by the left hand; and second, the point where the bow was drawn across the edge. As the slate was subdivided into thirty-six compartments, by either one of which it could be held, and as there was a corresponding point, across which the bow could be drawn, there were seventy-two primitive sounds that might be produced by means of this simple contrivance. Each of these sounds inherently and necessarily produced a different figure upon the slate, and there were consequently just seventy-two initial letters in the Aztec alphabet. The mode of instruction was extremely simple. A word was pronounced by the aged teacher at the front of the stage, written upon his slate, exhibited to the scholar at the black tablet, and by him copied upon it. The whole class then drew down their bows, so as to produce the proper sound, and the word itself, or its initial letter, was immediately formed upon the slate. After the seventy-two primitive letters or sounds had been learned, the next step was the art of combining them, so as not only to produce single words, but very often whole sentences. Thus the first hieroglyphic carved upon the tablet, on the back wall of the altar, in Casa No. 3 (forming the frontispiece of the second volume of Stephens's Travels in Central America), expresses, within itself, the name, date of birth, place of nativity, and parentage, of _Xixencotl_, the first king of the twenty-third dynasty of the Aztecs. The hieroglyphics of the Aztecs are all of them both symbolical and phonetic. Hence, in almost every one we observe, first, the primitive sound or initial letter, and its various combinations; and, secondly, some symbolic drawing, as a human face, for instance, or an eagle's bill, or a fish, denoting some peculiar characteristic of the person or thing delineated. But to return to the Hall of Students. The men and women on the stage were placed there as critics upon the pronunciation of each articulate sound. They were selected from the wisest men and best elocutionists in the kingdom, and never failed to detect the slightest error in the pronunciation of the tutor.