space consists of invisible steam : the white cloud beyond is not steam at all but consists of small particles of liquid water. The steam is condensed partly by coming in contact with the colder atmosphere and partly by the cooling effect due to its own expansion. The steam inside the kettle is under a somewhat greater pressure than that of the atmosphere; and when it gets outside it expands. Now, whenever steam or air expands it does work as truly as if it were expanding in the cylinder of a steam-engine or air-engine: and, as a supply of heat is thereby taken from it (corresponding to the work done), it is cooled by its own expansion. EXPT. 71. On the plate of an air-pump place a well-fitting bell-jar and work the pump rapidly. After about half a dozen strokes the air inside the bell-jar generally becomes misty. By the rapid exhaustion (and expansion) the air is cooled, and, if the temperature falls below the dew-point, some of the watervapour in it is condensed in the form of small liquid particles which form the mist. There is usually enough water-vapour present in the air to show this effect. 97. Rain, etc.-The particles of water forming a cloud tend to coalesce into larger drops, and, if these increase in size owing to further condensation, they presently fall as rain. Snow is produced when the temperature of the air falls below o°, and under the microscope is seen to consist of small hexagonal crystals (Fig. 68). The causes that favour the fall of rain are similar to those by which clouds are formed, viz. the cooling of warm, moist air (1) by coming in contact with colder air or cold land, and (2) by its own pansion. This latter cause comes into play whenever ex Fig. 68.-SNOW-CRYSTALS. warm air rises upwards as a convection-current, or when it is forced to ascend, as in passing over a range of hills; for as the air rises it gets into regions where the pressure is less (Arts. 29 and 51), and therefore it expands. In our own island the rainfall is heaviest on the western coasts, for the S.W. winds, having passed over the warm waters of the Gulf Stream, are themselves warm and moist: when they come in contact with the land, and especially when they have to pass over mountain-ranges, they become cooled and deposit their moisture as rain. Rainfall is measured by means of an instrument called a rain-gauge (Fig. 69). This consists of a funnel for collecting the rain and a graduated vessel for measuring its volume. This volume, divided by the area of the funnel, gives the depth of the layer of water which would be produced if all the rain remained on the surface of the ground. In Great Britain the average annual rainfall is somewhat under 30 inches. The driest district is Lincolnshire, where the rainfall is only about 20 inches. The rainfall is heaviest in the Lake District, the wettest place being the Stye in Borrowdale (Cumberland), where the rainfall is about 175 inches. ANSWERS TO EXAMPLES CHAPTER II (p. 13) 1. 7°.5 C. 2. 15° C.; 40° C.; 85° C. ; 95° C. 3. 194° F.; 176° F.; 86° F.; 23° F. 4 -37°-3 F.; 674°.6 F CHAPTER III (p. 21) 3. 40.048 ft. 4. 1.224 ft. 5. 22.128 cm. 150°. 7. 0.0432 in. 1. 0.00002797. 7. 100.24 c.c. 4. 320 c.c. CHAPTER IV (p. 26) 6. 2.0068 m. ; 2. 0.000302. 3. 0.0001561. 4 131°.7. CHAPTER V (p. 36) 5. 273°. 6. 91°. 7. 12. 2.38 litres; 13.2 litres. 8. 0.0036. 9. 546°. 10. 0.9462 gm. 11. Reduced to twothirds. 12. 75 c.c.; o° C. 13. 333°. 14. Volume remains unaltered. 15. 383.2 c.c. 16. 115.5 cub. in. 17. 273. CHAPTER VI (p. 46) 3. 3.1. 1. 76,800 units. 2. 1995 units. 4. 0.2. 5. 88.1. 6. 90°.9. 7. 17°. 9. o.1327. 10. 853°. |