The _Pneumatics_ consist of some seventy-eight theorems or experiments or tricks, call them what you will, which in different manuscripts and editions are variously grouped in a single book or two books. The same idea or method, however, is often repeated in the different chapters. Thus we encounter over half a dozen times the magic water-jar or drinking horn from which either wine or water or a mixture of both can be poured, or a choice of other liquids. And in all these cases the explanation of the trick is the same. When the air-hole in the top of the vessel is closed so that no air can enter, the liquid will not flow out through the narrow orifice in the bottom. Changes are rung on this principle by means of inner compartments and connecting tubes. Different kinds of siphons, the bent, the enclosed, and the uniform discharge, are described in the opening chapters and are utilized in working the ensuing wonders, such as statues of animals which drink water offered to them, inexhaustible goblets or those that will not overflow, and harmonious jars. By this last expression is meant pairs of vessels, secretly connected by tubes and so arranged that nothing will flow from one until the other is filled, when wine will pour from one jar and water from the other. Or when water is poured into one jar, wine or mixed wine and water flows from the other. Or, when water is drawn off from one jar, wine flows from the other. Other vessels are made to commence or cease to pour out wine or water, when a little water is poured in. Others will receive no more water once you have ceased pouring it in, no matter how little may have been poured in, or, when you cease for a moment to pour water in and then begin again, will not resume their outpour until half full. In another case the water will not flow out of a hole in the bottom of the vessel at all until the vessel is entirely filled. Others are made to flow by dropping a coin in a slot or working a lever, or turning a wheel. In the last case the vessel of water is concealed behind the entrance column of a temple. In one magic drinking horn the flow of water from the bottom is checked by putting a cover over the open top. When another pitcher is tipped up, the same amount of liquid will always flow out. [Sidenote: Various automatons and devices.]