Science for the School and Family, Part I. Natural Philosophy

by Worthington Hooker

Fig. 200.

Fig. 201.

284. Winds as Affected by the Rotation of the Earth.—The heat of the vertical sun upon the tropics causes a rise of heated air into the upper regions, while there is a rush of colder air toward the equator from both north and south. This effect is represented in Fig. 200 (p. 221), E being the sun, N the north pole, and S the south pole. An effect similar to that represented in Figs. 198 and 199 is produced here, but it is on a much larger scale. But the diagram does not present the matter in its true light in all respects. The prevailing winds in the equatorial regions are not north and south winds, as would appear from this diagram; but they are from the northeast and southeast. I will explain this by Fig. 201. As the earth turns on its axis it is plain that there is no part of the surface of the earth that moves so rapidly as the equator, E W, for that moves in a larger circle than any other part. And the nearer you go to either pole, N or S, the less is the rapidity of the revolution. Now the atmosphere, as stated in § 188, partakes of the motion of the earth. The air, therefore, at the equator is moving from west to east with the rotation of the earth faster than it is any where else, and the nearer you go to either pole the slower is its motion. It follows from this that any portion of air blowing from the north or the south toward the equator, as it comes from where it was moving east slower than air at the equator is, would from its lesser momentum lag behind the air of the equator, the wind would be curved toward the westward, as indicated by the arrows. The result would be that the northern wind would be converted into a northeaster, and the southern into a southeaster. All this can be made more clear with a globe, or, indeed, with any round object.

285. Liquefaction.—The change of solids into liquids is one of the most observable effects of heat. This change requires different degrees of heat in different substances. Thus while iron melts at the high heat of 2786°, lead melts at 633°, sulphur at 239°, ice at 32°, and mercury at 39° below zero. Mercury is never found in a solid state, but it sometimes becomes solid in the arctic regions when carried there and exposed in the open air. We are apt to think of water as being in a more natural state when liquid than when it is solid, just as we think of iron as being naturally solid and mercury as naturally liquid. But in all these cases the state of the substance depends on its temperature, and this is varied by circumstances. Water at the equator is always liquid, and the idea of ice there is exceedingly unnatural; while near the poles it is the reverse, ice and snow reigning every where throughout the whole year.

286. Evaporation.—There are two ways in which the change of a liquid into a vapor occurs. One is a rapid change when heat is so applied as to raise the liquid to its boiling point. This is commonly termed vaporization. The other mode is the ordinary gradual evaporation which goes on from the surface of the liquid. This process is going on continuously, not requiring any particular degree of heat, but occurring under all degrees of the temperature of a liquid. Its rapidity, however, is in proportion to the degree of heat, as may be seen by the rise of vapor from water that is being heated, long before it begins to boil. The same thing can also be seen in a bright summer's morning, when the heat of the sun causes the moisture gathered from rain or dew to rise so abundantly from fences, and boards, and roofs as to be visible like smoke.

287. Solution of Water in Air.—Evaporation is constantly going on from every wet surface, except when the air is so loaded with moisture that it can take up no more. The vapor is not ordinarily visible, the particles of water passing quietly upward among those of the air, being dissolved in the air just as some solids are dissolved in water. It becomes visible only when so much of it rises that the solution of the water in the air is not readily effected. The readiness with which the solution takes place depends much upon the temperature of the atmosphere. Some very common phenomena illustrate this. In a very cold day the breath of animals, as it comes out of the mouth, seems to be loaded with moisture. Why? It is not because there is more moisture in it than in warm weather, but because cold air can not hold in solution so much water as warm air can. The same explanation applies to the smoking of wet fences and roofs in the sun of a summer's morning. The moisture is heated by the sun, but the air, not having become very warm as yet, can not readily dissolve all the moisture that rises. The phenomenon is not apt to occur when the hot sun shines after a shower at mid-day or in the afternoon, because then the air is warm enough to take up all the moisture that is sent up into it.

How water, being heavier than air, rises in the atmosphere is a mystery. It has been supposed by some that it was owing to a kind of affinity existing between water and air. But in opposition to this is the fact that evaporation takes place more rapidly under the exhausted receiver of an air-pump, where there is almost no air, than it does where it is freely exposed to the atmosphere.

288. Clouds.—The water which goes up in the air in evaporation is variously disposed of. Some of it is deposited as dew or frost. Some of it forms fog. Some of it also mounts far upward and forms the clouds, which are really collections of fog made high up in the air. In fog and in clouds the water which in its evaporation is invisible becomes visible. Let us see how this is. There is always more or less of water in clear air, but the particles are so minutely divided and so thoroughly mingled with the particles of the air that they can not be seen. But in a fog or cloud the particles of water are gathered together in little companies, as we may express it. And it is supposed, some think ascertained, that each of these companies of particles is globular and hollow. If so, then we may regard every cloud as a vast collection of minute bubbles or balloons careering through the air.