Atmometer

Atmometer, an instrument which can be used to determine the humidity of the atmosphere. Its action has not yet been fully investigated. It consists of a hollow ball of unglazed clay with a glass stem. The whole is filled with water and inverted in a dish of mercury. As the water, having passed into the pores, evaporates from the surface of the ball, the mercury rises in the stem. If much water-vapour be present in the atmosphere, condensation takes place in the pores, and the mercury falls in the tube.