164. =Air-Guns and Pop-Guns.=--These illustrate the elasticity of condensed air. The air-gun is constructed in this way: A receiver, like V, Fig. 109, is made so that you can screw it on and off from the instrument. After being charged with condensed air it is screwed upon the gun, its stem communicating with the barrel. In order to discharge the gun there is a contrivance connected with the trigger for raising the valve, G, so that some of the condensed air may enter the barrel. On doing so, it by its sudden expansion rapidly forces out the contents. The principle on which the common pop-gun operates is the same. There is air confined between the two corks, _a_ and _b_, Fig. 111 (p. 123). As the rod, R, is pushed quickly in, the cork _b_ is carried nearer to _a_, so that the air between them is condensed. With the condensation the expansive force is increased; and when it becomes so great that the cork _a_ can no longer resist it, it throws the cork out, and so quickly as to occasion the popping sound. 165. =Powder and Steam.=--The explosion of powder furnishes a good illustration of the expansive force of condensed air or gases. These gases are produced so suddenly from the powder that at the instant they are in a very condensed state, and therefore expand powerfully. So, also, steam has power in proportion to its condensation. When formed under the confinement of a boiler, on being allowed to escape it expands with great force. The application of the expansive power of steam will be treated of particularly in another part of this book. [Illustration: Fig. 112.] 166. =Retardation by Condensed Air in Gunnery.=--When a ball is fired it is constantly retarded in its flight by the resistance of the air, for it has to push the air away on every side in order to make its way through it. Of course, then, the more condensed the air is the greater is the resistance. Now it is condensed air that the ball is obliged to remove; for as it goes forward it, by its rapid pressure, condenses the air directly before it. And the more rapid is its flight the greater is the condensation, and therefore the greater the resistance. Besides, the retarding effect is increased by the tendency to a vacuum behind the ball. All this can be made clear by Fig. 112. Let B be a ball going very rapidly in the direction indicated by the arrow, the cloud representing the condensed air before it, and the space included in the two lines the vacuum behind it. It is obvious that the more rapidly the ball goes the less readily is the air pressed out of the way, and therefore the more it is condensed in front of the ball. At the same time the more rapid is the ball the less readily does the air close up behind it, and therefore the greater is the tendency to a vacuum there. For these reasons there is more retarding influence exerted by the air upon a ball in the first part of its course than in its latter part. [Illustration: Fig. 113.] [Illustration: Fig. 114.]