1439. A hollow cylinder forms one of the best constructed rain-gauges; it has within it a cork ball attached to a wooden stem (fig. 247.), which passes through a small opening at the top, on which is placed a large funnel. When this instrument is placed in the open air in a free place, the rain that falls within the circumference of the funnel will run down into the tube and cause the cork to float, and the quantity of water in the tube may be seen by the height to which the stem of the float is raised. The stem of the float is so graduated as to show by its divisions the number of perpendicular inches of water which fell on the surface of the earth since the last observation. After every observation the cylinder must be emptied.