The Atmometer is merely a hollow ball of unglazed clay, to which a glass tube is luted. The whole is filled with boiled water and inverted so that the open end of the tube stands in a dish of mercury. The water evaporates from the outer surface of the clay (at a rate depending partly on the temperature, partly on the dryness of the air) and in consequence the mercury arises in the tube. In recent experiments this rise of mercury has been carried to nearly 25 inches during dry weather.