A force pump is usually provided with an air chamber which is connected with the discharge pipe. On the down stroke of the piston, water is forced into the air chamber. This compresses the air it contains. The compressed air reacts and exerts pressure on the water forcing it out in a steady stream. Force pumps are used in deep wells, being placed at the bottom. The pumps used in city water works, fire engines, and all steam pumps, are force pumps. (See Fig. 44.) [Illustration: FIG. 44.--A steam pump used on a fire engine.] =64. The Siphon.=--The siphon is a tube used to convey a liquid from one level over an elevation to a lower level by atmospheric pressure. It is used to remove liquids from tanks or vessels that have no opening at the bottom. The siphon cannot be completely understood until one has mastered the laws of the flow of liquids. The following is offered as an incomplete explanation of its behavior. Consider the siphon to be full of water and closed at _d_ (Fig. 45). Atmospheric pressure on _a_ will hold the siphon full if _ab_ does not exceed 34 feet. If _d_ is opened the water falls out with a speed equal to that acquired in falling from the level of _a_ to that of _d_. This speed is acquired by all the water in the siphon and results in a drop in pressure throughout it. The pressure at _a_ inside the siphon becomes less than the pressure at the same level outside as soon as the water starts flowing. The water in the vessel then flows into the siphon and out at _d_. This flow continues as long as there is a fall from the free surface of the water in the vessel to the outlet at _d_. [Illustration: FIG. 45.--Cross-section of a siphon.] [Illustration: FIG. 46.--The Cartesian diver.] =65. The Cartesian Diver.=--This is a device which illustrates at the same time transmission of pressure by liquids, Archimedes' principle, and compressibility of gases. It was invented by Des Cartes (1596-1650). As ordinarily made, it is a hollow glass image with a small opening in the foot. It contains air and water in such amounts that the average density of image and contents is slightly less than that of water. It is placed in a tall glass jar filled with water and covered with tightly stretched rubber tissue. (See Fig. 46.) By pressing on the rubber cover the diver may be made to sink, since the air and water transmit the pressure on the cover which compresses the air inside the figure admitting some water to it, thus making the diver more dense than water. By varying the pressure it can be made to sink, rise, or remain stationary at will.[D] A small vial can be used instead of the image. [D] The position of a submarine in or under water is controlled in a similar manner.