Around the chromosphere is another layer of flame which extends for hundreds of thousands of miles in all directions. This last layer is called the _corona_, and it is as thin as the stuff of which dreams are made. It is formed of the gas coronium and since it is so thin it can never be seen except when there is a total eclipse, that is, when the Moon passes between us and the Sun, which will be explained in Chapter VII, and so shuts out the intense light of the other two layers. A cross section of the Sun is shown in Fig. 33. [Illustration: FIG. 32.—PROMINENCES OF THE SUN COMPARED WITH THE SIZE OF THE EARTH.] =Sun Spots and Their Effect on the Earth.=—Very often great spots are seen on the Sun’s surface. These purplish black spots appear to be holes, like the craters of volcanoes, in the photosphere, or layer of flame next to the core of the Sun. The sun spots are caused by great eruptions which take place in the core of the Sun, and these sun spots are sometimes over 100,000 miles in diameter, when they can be easily seen with the naked eye; indeed if a sun spot has a diameter only as large as that of our Earth, it can be seen with the naked eye, protected, of course, with either a smoked or a colored glass, or better, with a pinhole telescope; in any case it will look like a black speck about the size of a pinhead. Fig. 34 shows a view of a sun spot made through a large telescope. [Illustration: FIG. 33.—CROSS SECTION OF THE SUN.] Whenever the sun changes his spots _magnetic storms_ take place on the Earth, when compass needles, telegraph and telephone apparatus and wireless systems are disturbed. When a large number of spots appear at the same time on the Sun the _Northern Lights_ are often very bright. Sun spots have something to do with our weather, but the effects are not yet well understood. =The Sun and the Weather.=—Of course the Sun has everything to do with the weather, but to be able to predict the kind of weather we shall have even the next day is a very hard thing to do. The changes in the weather are caused by the heat of the Sun alone. The heat of the Sun produces clouds by vaporizing the water of rivers, lakes and oceans. He causes hot and cold weather by heating some parts of the air more than other parts, and this sets the air in motion and we call this movement of the air the wind. [Illustration: FIG. 34.—SUN SPOT IN PHOTOSPHERE.] Changes of heat, moisture and wind are the cause of all the kinds of weather we have, and we have a good many kinds, be it hot or cold, dry or wet, calm or windy, clear or stormy, good, bad and indifferent. =To Forecast the Weather by a Barometer.=—The best way to tell what the coming weather will be in the next few hours is by the rise and the fall of the _pressure_ of the air, or _barometric pressure_, as it is called.