Polarization (wave propagation)

“ Since vorticity in a perfect fluid cannot be created or destroyed, the modification of the system which is to be analogous to an electric field must be a polarized state of the vortex motion, and light must be represented by a communication of this polarized motion from one part of the medium to another. Many distinct types of polarization may readily be imagined: for instance, if the turbulent motion were constituted of vortex-rings, these might be in motion parallel to definite lines or planes ”

“ The Ionosphere and Radio Waves When a radio wave encounters a particle having an electric charge, it causes that particle to vibrate. The vibrating particle absorbs electromagnetic energy from the radio wave and radiates it. The net effect is a change of polarization and an alteration of the path of the wave. That portion of the wave in a more highly ionized region travels faster, causing the wave front to tilt and the wave to be directed toward a region of less intense ionization. ”

“ As a matter of fact, rapid changes in sky polarization indicate about the only thing that can be authoritatively asserted—namely, that the invisible particles behave much like the molecular constituents of the air. ”

“ As soon as one wave passes, another, of the reverse polarity, always succeeds. I have never known this to fail, and it may be set down as an invariable rule. When the poles of the aurora are in unison with the poles of the current upon the line, its effect is to increase the current; but when they are opposed, the current from the battery is neutralized,—null. These effects were observed at times during Saturday, Saturday evening, and Sunday, but were very marked during Sunday evening. ”

“ From the fact that one half of the atmosphere, including nearly all of its water vapor, lies below an elevation of three and one half miles, it becomes evident that the greater part of the absorption of the sun’s rays must take place in the lower strata. On clear days the atmosphere absorbs nearly one half of the sun’s heat rays; the remainder reaches the surface of the earth, warms it and in turn is radiated back into the air,—with this difference: that as earth radiation the wave motion of the rays is longer and slower than it was when the rays entered our atmosphere as solar radiation. ”

“ When the wave brought by the polar currents arrives—coming as it does from regions where it naturally has less velocity—it does not at once acquire the velocity of the earth’s motion at the equator; and since the rotation of the earth is from west to east, this portion of the water lagging behind forms a stream or current which has an apparent motion from east to west, that is to say, apparent as regards the earth, but real in relation to the adjacent land and water. The trade winds, which in this zone blow constantly in the same direction, lend their aid in maintaining the equatorial current. ”

“ On this basis we should expect that in the driest air at great heights, where the temperature is low and condensation has but just begun, and the dust particles are rare, there would occur the smallest aqueous particles reflecting light of the feeblest intensity but the largest percentage of polarization. Rayleigh has shown that it is quite possible that the molecules of oxygen and nitrogen constituting the atmosphere may also exercise a diffuse selective reflection, and contribute to the brightness and polarization that are mainly due to aqueous vapours. ”

“ The energy of any polarized state of vortex-motion would be greater than that of the unpolarized state; so that if the motion of matter had the effect of reducing the polarization, there would be forces tending to produce that motion. ”

“ Thus, without wind, we should have a perpetual and uniform system of tropical and polar currents; though without wind, Sir John Herschel maintains, [9] we should have no "considerable currents" whatever in the sea. In consequence of the diurnal rotation of the planet on its axis, each particle of oil, were resistance small, would approach the poles on a spiral turning to the east with a relative velocity greater and greater, until, finally, it would reach the pole, and whirl about it at the rate of nearly a thousand miles the hour. ”

“ To compensate this flow, air rushes on the earth's surface toward the equator, and as it also is deflected from its course by the same inertia of rotation, it appears in the tropics as a trade wind blowing from the northeast. However, the upper warm current does not flow all the way to the pole in the upper regions; it is gradually cooled down, and in about the thirtieth degree of latitude it begins to descend to the earth's surface, where it meets with the cold polar current. ”

“ On comparing the percentage of light polarized and the angles of incidence it is seen that they decrease together. No definite maximum was made out, but the existence of one near or beyond 60° is perhaps indicated by the fact that polarization was less easily observed on the evenings previous to June 29, and by the more rapid variation in the percentages on this and the two succeeding evenings. ”

“ Hence it would require only one sixth as long to raise the temperature of 1 gram of water 1° C. In other words, assuming that, in the meantime, all the incoming radiations are absorbed and that no heat is lost by conduction, convection or radiation, then it will require the radiations from Polaris to fall upon 1 square centimeter continuously for one million years in order to raise the temperature of 1 gram of water 1° C. In marked contrast with this value, the radiation from the sun which is transmitted by our atmosphere and falls upon an area of 1 sq. ”

“ But some steps have recently been made in thermotics, which appear to be likely to overturn this belief, and to make the doctrine of emission as untenable with regard to heat, as it had been found to be with regard to light. I speak of the discovery of the polarization of heat. It being ascertained that rays of heat are polarized in the same manner as rays of 154 light, we cannot retain the doctrine that heat radiates by the emanation of material particles, without supposing those particles of caloric to have poles; an hypothesis which probably no one would embrace ”

“ It can, therefore, hardly be matter of doubt, that great permanent currents, caused by the unequal heating of the equatorial and polar regions, do exist in the higher strata of the atmosphere—an inference which is supported not only by the occurrence of the trade-winds and the monsoon, but by a variety of other facts and observations. ”

“ Assuredly we cannot have a current from the poles without a contrary current to the poles. If we go into the arctic circle, we again find the westerly and northerly winds predominating. If the current from the equator follows the surface, the westerly winds ought to be south-west. If it be above the surface wind, then the surface wind is the polar current, and ought to be north-east. ”

“ The polarization of the light transmitted by the pile is never complete, but tends to become more nearly so as the number of the plates is increased and at the same time the angle of incidence for which the polarization is a maximum approaches indefinitely the polarizing angle (Sir G. G. Stokes, Math. and Phys. Papers, iv 145) .
In order to isolate a polarized pencil of rays with a rhomb of Iceland spar, it is necessary to have a crystal of such a thickness that the emergent streams are separated, so that one may be stopped by a screen. ”

“ In clear weather we find the air near to the earth in a neutral condition, but gradually assuming the condition of a positive charge as we ascend; so that the upper air and the earth are oppositely charged like the two sides of a Leyden jar or two leaves of a condenser. This condition is intensified and localized when a thunder-cloud passes over the earth. The moisture globules have been charged with potential energy by the power of the sun's rays when evaporation took place ”

“ As a radio wave passes into the ionosphere, it loses some of its energy to the free electrons and ions present there. Since the amount of absorption of the radio-wave energy varies with the density of the ionospheric layers, there is no fixed relationship between distance and signal strength for ionospheric propagation. Absorption fading occurs for a longer period than other types of fading, since absorption takes place slowly. Under certain conditions, the absorption of energy is so great that communication over any distance beyond the line of sight becomes difficult. ”

“ Even the apparently quiet cloud is always in motion within itself. Rising currents of moist air, chilled by its own expansion, cause condensation of the vapor into cloud matter. The coalescence of cloud matter into mist and droplets results in their fall to a lower level, where they are again vaporized; and the vapor, in turn, rises in the updraught. All this is constantly changing and disturbing the electric potential. When, however, the updraught is strong enough to shatter the drops into mist, the potential becomes so high that the violent discharges constitute the thunder-storm. ”

“ Now, the more even the distribution of temperature on the ground the less the probability of coarse interconvection, and the same is true of any higher stratum of air, provided it is free from disturbing influences from outside. If, therefore, we have large currents near the ground, ending, as they must, in cumulus, it has already been explained that these clouds stop the action, and the general system of large currents will be restricted to the region in which they occur. ”

“ The inequality of the solar action on the land and water, together with the tendency of the atmosphere to preserve an uniform density, is the cause of these periodically shifting currents. During the day the land acquires a temperature higher than that of the ocean, and the air over it is therefore rarefied and ascends, and the cooler air from the sea glides in to fill the partial vacuum produced. ”

“ When these moisture globules condense into drops of water the potential energy is set free and becomes active either as heat or electricity. The cloud gathers up the energy into a condensed form, and when the tension gets too great a discharge takes place between the cloud and the earth or from one cloud to another, which to an extent equalizes the energy.
In most cases of thunder and lightning it is only a discharge from cloud to cloud unequally charged. This does not relieve the tension between the earth and the cloud, but distributes it over a larger area. ”

“ The second and third of the observations of July 1, in the list below, were made upon regions removed several degrees from the nucleus, but though the amount of polarization is somewhat less, and tends to confirm the above conclusion, the difference is hardly greater than would be accounted for by the errors of observation. Determinations of polarization at great distances from the nucleus were not possible, the light being too feeble. ”

“ Solar energy may seem to be propelled in waves, but that is due to the action of coexistent and diverse influences. A given form of organized energy does not proceed in waves but in direct lines. The presence of a second or a third form of force-energy may cause the stream under observation to appear to travel in wavy formation, just as, in a blinding rainstorm accompanied by a heavy wind, the water sometimes appears to fall in sheets or to descend in waves. ”

“ There is, therefore, a constant tendency with the air that these upper currents carry polarward to be crowded out, so to speak—to slough off and turn back. Thus the upper current is ever ready to supply the trade-winds, as they approach the equator, with air exactly at the right place, and in quantities just sufficient to satisfy the demand.
224. How is it drawn down from above?—This upper air, having supplied the equatorial cloud-ring (§ 514) with vapour for its clouds, and with moisture for its rains, flows off polarward as comparatively dry air. The dryest air is the heaviest. ”

“ Currents of cool vapor descend toward the surface, after assuming, no doubt, vorticose motions, and sweeping away over wide areas the brighter cloud-masses, so as to form dark spots on the disk of the planet. And, owing to the various depths to which the different cloud-masses belong, and whence the up-rushing currents of heated vapor have had their origin, horizontal currents of tremendous velocity exist, by which the cloud-masses of one belt or of one layer are hurried swiftly past the cloud-masses of a neighboring belt, or of higher or low cloud-layers. ”

“ It is produced by the obliquely upward movement of the tropical current over the polar current, and by its rising more or less vertically in the vicinity of contact, after its horizontal progress northward has been checked by encountering the polar current. This oblique and upward movement of the tropical current diminishes the atmospheric pressure there, as shown by the barometer, and produces that depressing calm which is always felt by persons in any locality where this meeting of currents takes place, or over which its area moves or oscillates during the continuance of a storm. ”

“ The near neighbourhood of the ground is not essential. As long as the temperature of the air at any level is rising, so long interconvection must occur. The process will be independent of the presence or absence of wind. All that wind can do is to mix up the air at different levels, breaking the system of currents and reducing it to, so to say, a finer texture, or producing eddies, if strong enough, which direct the currents and gather them into definite channels. The final result in any case is that, with rising temperature, water vapour is steadily borne upwards from the ground. ”