There are two kinds of fountains produced by the elastic pressure of the air. One consists in compressing the air in a close vessel, and causing it to act on the surface of the water as in the force pump. The other is represented in the annexed figure, and is an apparatus commonly employed at lectures for illustrating the properties of air. a the glass receiver, in which the fountain is seen to play, which must be made to fit air-tight to the ground brass plate b beneath. The stop-cock c must then be screwed into the plate of an air-pump, and a vacuum formed. After turning the stop-cock to prevent the entrance of air, the lower extremity should be immersed in a vessel of water; and on again opening the communication, the pressure of the atmosphere on the surface of the water will cause it to rise in a stream, and form a beautiful jet-d'eau.