42. WARM, FAIR WEATHER FOLLOWED BY RAIN OR SNOW 48. MANNER IN WHICH THE COLORS OF THE RAINBOW ARE 59. ST. ELMO'S FIFE-ON TOP OF SPIRE OF NOTRE-DAME 60. NORDENSKJÖLD'S. CHART OF ISOCHASMIC LINES 61. ELECTRIC EGG 292 295 296 62. DISCHARGE IN VACUUM : 297 64. 65. 66. 63. LUMINOUS DISCHARGE THROUGH TORRICELLIAN VACUUM DISTRIBUTION OF IRON FILINGS ON A STRAIGHT BAR MAGNET DIP OR INCLINATION OF THE MAGNETIC NEEDLE 298 304 307 309 315 315 315 PROPERTY OF THE CITY OF NEW YORK THE WONDER BOOK OF THE ATMOSPHERE CHAPTER I THE SHAPE, HEIGHT, AND COMPOSITION OF THE ATMOSPHERE The word "atmosphere" means literally a globe or sphere of air. If, however, you have the idea that the atmosphere, or the ocean of air that surrounds our earth, has the shape of a huge sphere you will be mistaken, for our atmosphere is only a thin shell or layer of air, the lower surface of which takes the shape of the earth's surface, on which it rests, while even its upper surface is not that of a true sphere, but of an oblate spheroid. A A spheroid is a ball or sphere that is slightly flattened at its opposite sides. When a spheroid rotates, or spins like a top, the diameter on which it turns is called its axis. In Figs. 1 and 2 two different kinds of spheroids are represented. In that shown in Fig. 1, the diameter AB, on which the spheroid is assumed to rotate, is shorter than the diameter B FIG. 1. OBLATE SPHEROID D CD; or, as we would say in geography, the polar diameter AB, is shorter than the equatorial diameter CD. Such a spheroid is called an oblate spheroid. Fig. 2, represents a spheroid in which the axis or polar diameter AB, is longer than the equatorial diameter CD. Such a spheroid is called a prolate spheroid. We do not know accurately the height of the earth's atmosphere. We are certain, however, that this height C A D cannot exceed a certain value; for, we know just how fast the earth turns or rotates on its axis, and can, therefore, calculate its centrifugal force, or the force in a spinning body that tends to throw things off from its surface. Now, it can be shown, in the case of our earth, that as soon as the centrifugal force in the equatorial regions reaches a given value, the particles of air in the atmosphere that extend a certain height above the earth's surface, would no longer be able to remain on the earth, but would be thrown off into space, never again to return. In this manner by making a proper allowance for the decrease in weight, that all bodies undergo by reason of their increased distance above the surface of the earth, it is possible to show that the height of the atmosphere cannot be greater than 21,000 miles above the surface of the earth, since, at such a distance, no particles of air could remain on the earth. B FIG. 2. PROLATE SPHEROID It is by no means easy to determine the height of the earth's atmosphere; for we live at the bottom of this mass of air, and, since air is transparent, we cannot see its upper surface. Fortunately, however, there is an instrument that enables us roughly to measure the height of the atmosphere from the amount of pressure the air exerts on the earth's surface. This instrument is called a barometer and will afterwards be described in full. By its use, by noting the decrease in the pressure the air exerts at different heights above the level of the sea, it is possible to make a fairly close estimate of the height of the atmosphere, or at least the distance the greater portion of it lies above the earth's surface. Calculations made in this way seem to show that the least height above the level of the sea it is possible for the atmosphere to have, is somewhere between forty-five and fifty miles. It is almost certain, however, that a small portion of our atmosphere exists above these heights, and, although the distance to which it extends may be very great, yet the weight of this portion must be extremely small. While, therefore, the height of the atmosphere cannot exceed 21,000 miles above the level of the sea, its height must at least be as great as forty-five to fifty miles above such surface. There is another way in which the height of the atmosphere can be calculated, and this is by the duration of twilight. Had our earth no atmosphere whatever, daylight would not begin until the sun rose above the horizon, and night would set in the moment the sun sank below it. On the other hand, if our atmosphere extended for an unlimited distance above the earth's surface there would be no night; for, no matter where the sun might be, some of its light would be reflected to all parts of the earth's surface from some of its different layers. Now twilight increases the length of the day to an extent depending on the height of the atmosphere. After the sun has set, its rays light up the air over our heads and this lighted air reflects or throws the light down on the earth, so that the earth's surface remains lighted long after the sun has set, and begins to receive the light long before the sun has risen. While the duration of twilight varies in different parts of the earth, yet the night does not actually begin until the sun has sunk about 18° below the horizon. The astronomer, Young, shows that on March 1st, and October 12th, in lat. 40° N., it takes the sun about ninety minutes to reach this distance of 18° below the horizon, after it has just sunk below it, while on the 20th of June, it requires more than two hours for the sun to reach this position. In latitudes higher than 50°, where the lengths of daylight are the greatest, twilight never quite disappears, even at midnight. On the other hand, in elevated regions, such as in the high mountain districts of Peru, the length of twilight never exceeds thirty minutes. Calculated in the different ways above pointed out the height of our atmosphere may, perhaps, be taken as about 200 miles above the level of the sea. It most probably extends, however, in a very tenuous condition for considerable distances above this height. So far as its weight is concerned the greater part of the atmosphere lies within a few miles of the earth's surface. In the higher regions of our atmosphere the air must be exceedingly rare. Indeed, there is so small a quantity of air present that this space can almost be regarded as a vacuum. It is the passage of electric discharges through the air of these regions that produces auroras, as will be explained in the chapter on the aurora borealis. Here, too, occur most of the phenomena of meteorites, those wandering solid bodies that move so rapidly through the air that they are raised to a glowing heat by friction against the few particles of air in these regions. It is also in the higher regions of the atmosphere that many of the phenomena of magnetism occur. Moreover, it is probably from a portion of the atmosphere, situated somewhere between its lower or grosser portions and its higher or more tenuous portions, that the electro-magnetic waves, employed in wireless telegraphy, are reflected or thrown back to the earth, and are thus confined to the lower parts of the atmosphere, and prevented from being dissipated throughout the space that lies outside our atmosphere. |