Air, like water, seeks the level where the lowest pressure exists. It is 1,600 times lighter than water, and it extends to some 50 miles above the earth. One half of its weight is below the three-mile limit. Atmospheric pressure is variable, and the temperature of the air usually decreases with the altitude, so that it is often very cold up in the air when it is comparatively cold on the ground. For that reason electrically heated clothing or cabins, heated from the engine, are used to keep the pilot and passengers warm. The change in the temperature of the earth sets the air in motion, so that portions that are heated by the sun’s rays faster than other portions affect the atmosphere more quickly in that locality than in others, for the heated air rushes up by expansion and the cooler air will rush into the vacated place. With the repetition of this the movement of the air increases. Thus high-pressure areas and low-pressure areas are formed. A glance at a United States Weather-Bureau map will show the location and the atmospheric pressure at various places in the United States, and the intelligent reading of the same will be of infinite usefulness to the aviator. The atmospheric pressure is measured by a barometer. It is measured by a column of mercury necessary to balance it. This same atmospheric pressure is used to operate the altimeter, which tells the aviator how high he has climbed. A falling barometer indicates the approach of a storm and a rising barometer fair weather. Wind strength is usually indicated by miles at which the storm is raging. In the early days of aviation the aviator used to wet his finger to see if the wind was stirring and what quarter it was from. If it was blowing many miles an hour, he would not venture forth. In starting or landing a machine it is always desirable to head into the wind. It is true that in forced landings pilots have come down with the wind, but for every foot they must make an allowance. Atmospheric pressure also has much to do with the flying efficiency of the wings. The heat generated on the surface of the planes used by the United States army in Mexico caused the dope to peal in some cases and rendered the planes unfit to fly. The flier should, however, know something about the kinds of winds which prevail and the times of the day when the most violent are to be encountered. At the earth’s surface the day winds are stronger than the night winds, and the average velocity of the day wind is about eleven miles an hour. Because of the similarity of the movements of the winds to those of water, many of the terms applied to air movements are the same.