Abstract: An enclosed container has a chamber in which a gaseous fluid is heated as by solar radiation and a first valve permits some of the fluid to be ejected as it expands during heating. The remaining fluid is then cooled as by clouds blocking the sun or during the night and a subatmospheric pressure is developed in the chamber. Through a second valve in the container this negative pressure is communicated to a siphon having an inlet immersed in liquid at an upper level and an outlet closed by a third valve at a lower level. The subatmospheric pressure causes the siphon to be filled with liquid and the weight of the liquid opens the third valve at the outlet causing the siphon to operate. The second valve prevents the loss of subatmospheric pressure in the siphon. The siphon may be used to automatically drain water from areas such as flat roofs of buildings.