A simple _barometer_ for showing the changes in the pressure of the air can be made of a glass tube about 3 feet long, ½ inch in diameter, and closed at one end as shown in Fig. 35. Fill the tube with mercury and, placing your finger over the mouth of the tube, turn it upside down and put the open end into a cup, or other vessel, which is half full of mercury; in placing it into the cup be careful that no air gets into the tube. Only a small part of the mercury in the tube will run into the cup and this will leave a space in the top of the tube. Now fasten a yardstick, the purpose of which is to show the changes in the height of the mercury in the tube, with a string or wire to the tube, and your barometer will be complete, as shown in Fig. 36. Since the air presses on the mercury in the cup but not in the tube, the pressure of the air on the mercury in the cup just balances the weight of the mercury in the tube and, hence, any increase or decrease in the pressure of the air, which ordinarily is about 15 pounds to the square inch, is shown by the rising or the falling of the mercury in the tube. The barometer in helping to forecast the weather shows that: (1) when the mercury _rises_ in the tube, that is when it is high, the weather will be _fair_; (2) when the mercury _falls_ in the tube, that is when it is low, _bad_ weather may be looked for; (3) when the mercury _suddenly falls_ in the tube a _storm_ is coming, and (4) when the mercury continues at a _high point_ the weather will remain _fair_. [Illustration: FIG. 35.—BAROMETER TUBE.] [Illustration: FIG. 36.—BAROMETER COMPLETE.] To remember these forecasts easily they may be briefly stated thus: (1) A _high_ barometer shows _fair_ weather. (2) A _low_ barometer shows _bad_ weather. (3) A _sudden fall_ of the barometer shows a _coming storm_. (4) A _constant high_ barometer shows continued _fair_ weather. =To Forecast the Weather by Signs.=—It will seldom happen that a boy who goes camping, or one who otherwise wants to know what the weather is likely to be on the morrow, will have a barometer to consult, so the next best thing is to know how to read the weather signs: (1) “Red at night is the sailor’s delight” is an old forecast, and means that the morrow will be a fine day. (2) “Red in the morning is the sailor’s warning,” which means that rain is coming. (3) A golden sunset is a sign that a high wind is coming. (4) A yellow sunset is a sign that rain is coming. (5) When the Sun sets clear it is a sign of a fine day on the morrow. (6) When the Sun sets behind a cloud it is a sign that the next day will be cloudy or rainy. (7) A misty dawn shows the coming of a fine day. (8) A low dawn, that is when the Sun shines clear on rising, shows the coming of a fine day. (9) A high dawn, that is when the Sun rises over a haze, or clouds, shows wind.