Our picture on the other side of this sheet shows the instruments used by the "co-operative" observers of the Weather Bureau. These observers, of whom there are about 4,500, well distributed over the country, serve the government without pay, and their painstaking observations have alone made possible a detailed survey of our climate. In the picture we see, on the right, an ordinary rain-gauge, and, on the left, a thermometer-screen containing two thermometers; viz., a maximum thermometer, for recording the highest temperature of the day, and a minimum thermometer, for recording the lowest. The screen, which is of wood, painted white, serves to shield the instruments from the rays of the sun, while permitting free ventilation. Under these conditions the thermometers show the temperature of the _air_; whereas when exposed to direct sunlight a thermometer shows the temperature acquired by the instrument itself, and this may differ materially from the air temperature. In contrast to this simple equipment, we find at a regular meteorological station, or observatory, an impressive collection of apparatus for observing and recording nearly all the elements of weather. The pressure of the air is measured by the mercurial barometer, and registered continuously by the barograph; the temperature of the air is automatically recorded by the thermograph. Other self-registering instruments maintain continuous records of the force and direction of the wind, the amount and duration of rainfall, the duration of sunshine, the humidity of the air, etc. There are also instruments for measuring evaporation, the height and movement of clouds, the intensity of solar radiation, the elements of atmospheric electricity, and various other phenomena of the atmosphere. [Illustration: A MAJESTIC CUMULUS CLOUD] CLOUDS AND RAINFALL Monograph Number Three in The Mentor Reading Course