. The mastery of water. looks to the mountains,to which the warm, moist winds give more generouslythan to the level plain. Air can take up and hold as invisible vapour a quantityof water which varies with the temperature. In warmweather the air is nearly always moister than in is to say, if you were to collect a cubic foot of airin winter and in summer at the same place, and measurethe amount of water which it contained, the latter wouldbe found to contain the most. Such an experiment isnot difficult to perform. Certain substances, like calcium 36 THE MASTERY OF WATER. chloride, afte
. The mastery of water. looks to the mountains,to which the warm, moist winds give more generouslythan to the level plain. Air can take up and hold as invisible vapour a quantityof water which varies with the temperature. In warmweather the air is nearly always moister than in is to say, if you were to collect a cubic foot of airin winter and in summer at the same place, and measurethe amount of water which it contained, the latter wouldbe found to contain the most. Such an experiment isnot difficult to perform. Certain substances, like calcium 36 THE MASTERY OF WATER. chloride, after being strongly heated to dry it, absorbswater very readily. If a bent tube, as shown in the figure,is filled with fragments of this material, and a measuredvolume of air is passed through it, the tube wiU extractthe water and consequently increase in weight. Asmoist air is lighter than dry air, care must be taken tofind the density (that is, the weight of unit volume) andfrom this to calculate the weight of air Culcwm Chloride- Shontng the yGlumeof air that has pass - C(7 through Ijtubc AN EXPERIMENT TO MEASURE THE AMOUNT OFMOISTURE IN AIR. When warm winds blow in from the sea they areheavily charged with moisture, and, if the land rises,they are compelled to curve upwards with it. In thisway they enter a region of lower pressure ; for, as isexplained in the Mastery of Air, the pressure on a moun-tain is almost invariably lower than at the , when the pressure on any portion of a gas is reduced,it expands and becomes cooler ; thus the moist air canonly retain a portion of the water vapour it excess is thrown out, and forms clouds, or, if thedrops are large enough, mist or rain ; or, if the temperatureis low enough, snow or hail. IMPOUNDING THE RIVER. 37 In a mountainous country, especially if it is not farfrom the sea, the summits are frequently wreathed inmist, and rain falls almost daily. Consequently, suchdistricts are the wettest regions of
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