This instrument consists essentially of a cryophorus, one bulb of which is kept wet by the same arrangement as is used for a wetbulb thermometer, while the other bulb is left dry. For convenience, the wet bulb is made spherical, and the dry bulb is made cylindrical, and is graduated. The difference of temperature between the two bulbs produced by evaporation causes a transference of water from the dry bulb to the wet bulb, and the amount of water thus transferred in any period of time, taken along with the mean temperature, will give a measure of the average hygrometric condition during that time. Preliminary observations with this instrument show that even in comparatively moist weather the total evaporation during twenty-four hours is great enough to be easily measured.