The pressure of the air upon the leather keeps it against the stone. With how much force does the air press the leather against the stone? With a force of fifteen pounds on every square inch. When there is air between the stone and the leather, can the stone be lifted with the sucker? It cannot; because the air under the leather will press it up as much as the air above will press it down. How does a fly walk upon the ceiling? The fly’s feet are much like the sucker, and are kept against the ceiling by the pressure of the air. How do snails cling to objects? They force out all the air between the object and their bodies, and are then held against the object, as the sucker is held against the stone. What is a _siphon_? A _siphon_ is a bent tube, having one branch longer than the other, as is represented in Fig. 15. [Illustration: Fig. 15.] For what is the siphon used? The siphon is used to draw liquids from one vessel into another. How can water be drawn from a vessel by means of a siphon? Put the short branch in the water, then, by means of the mouth applied to the long branch, suck the air from it, and the water will begin immediately to flow from the long branch. Where must the open end of the short branch be kept? It must be kept beneath the surface of the water, so as to prevent air from entering the tube. Where must the open end of the long branch be kept? It must be kept lower than the open end of the short branch. While the water is running out of the long branch, what is produced at the bend of the tube? The tendency is to produce a vacuum at the bend of the tube. What is the result of this? The water is continually forced up the short branch to fill this vacuum, so that it runs out of the long branch in a steady stream. How do boys often suck cider from a barrel? By means of a straw. How is this done? One end of the straw is placed in the cider, and the other end in the mouth; then the air is sucked out of the straw, and the cider is forced up to fill the vacuum. When the cider is sucked out, more cider is forced up, so that the straw is kept full. What makes the cider rise to fill the vacuum in the straw? The pressure of the air on the cider in the barrel. To what height will water rise in a tube to fill a vacuum? The water will rise to the height of thirty-four feet. To what useful purpose is this applied? It is applied to making _pumps_. Of what material are pumps made? Either of wood, or of metal, such as lead or iron. What is necessary in regard to the tube or pump-tree? It must always be air-tight. Why must the tube or pump-tree be air-tight? So that a vacuum may be produced in it. How is this accomplished? By means of two valves in the pump-tree. What is a valve like? A valve is like the lid on a tea-kettle, or the cover on an inkstand; and it is kept in its place by a hinge on one side. Where is one valve placed?