A form of hygrometer in common use is shown in Fig. 150. In this device, a thin strip of hygroscopic material (as a piece of goose quill) is formed into a spiral coil. One end of this is fastened to a post. The other end carried a hand or pointer. The latter moves over a printed scale and indicates directly the relative humidity. Its indications should be tested by comparing its readings with the results of dew-point determinations. The position of the pointer may be adjusted by turning the post. Important Topics 1. Water vapor in the air. Cause and effect. 2. Formation of dew, fog, rain, and snow. 3. Dew point, relative humidity. 4. Use of the dry- and wet-bulb hygrometer. Goose-quill hygrometer. Exercises 1. How is the relative humidity of the air affected by warming it? Explain. 2. How does the white cloud of steam seen about a locomotive in cold weather differ from fog? Explain. 3. In cold weather is the relative humidity of air out of doors and indoors the same? Explain. 4. Compare the relative humidity of air in a desert and near the ocean. 5. Look up the derivation of the term "hygrometer." Give the use of the instrument. 6. Find the relative humidity of air at 20°C. if its dew point is at 10°C. 7. How may the relative humidity of the air in a home be increased? 8. What is the effect of high humidity in the summer upon human beings? How do you explain this? 9. Does dew fall? Explain how dew is formed? 10. In what respects is a cloud similar to a fog? In what respects different? 11. Why are icebergs frequently enveloped in fog? 12. Does dew form in the day time? Explain. (7) EVAPORATION =173. Effects of Evaporation.=--In Art. 19 the cooling effect of evaporation is mentioned and some explanation is made of the cooling effect observed. Since evaporation is employed in so many ways, and since its action is simply explained by the study we have made of molecular motions and molecular forces, it may be well to consider this subject further. Take three shallow dishes, and place in one a little water, in another some alcohol, and some ether in the third, the liquids being taken from bottles that have stood several hours in the room so that all are at the same temperature. After a short time take the temperature of the three liquids. Each will be at a lower temperature than at first, but of the three the ether will be found to be the coolest, alcohol next, and the water nearest its first temperature. It will be noticed also that the ether has evaporated most in the same time. Similar effects may be observed by placing a few drops of each of these three liquids upon the back of one's hand, or by placing a few drops in turn upon the bulb of a simple air thermometer.