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Updated: June 21, 2025
Another peculiarity of this region, produced by the unequal rarefaction of the air over its different portions, is the occurrence, especially in spring and summer, of sudden gusts, hot or cold, which blow with great violence.
This force was then either the result of the tendency of an ethereal fluid to move from the free regions of space, where its density is a maximum, towards the planetary bodies around which there exists a greater degree of rarefaction, or the consequence of the impulsive force of some fluid medium.
The day had been fine, the evening was beautiful, but it was the rarefaction of the air immediately playing on the ground, and not the haze at sunset that caused what I have noticed. It is distinct from mirage, although it is difficult to point out the difference. The one, however, distorts, the other conceals objects, and gives them a false distance. The one is clear, the other is cloudy.
According to the wave theory, a condensation and rarefaction are necessary to constitute a sound wave. Surely, if a condensation is not produced, there can be no sound wave! We have then no need to consider anything but the condensation or compression of the supposed air molecules, which will shorten the discussion. The property of mobility of the air and fluidity of water are well known.
The reasoning was as follows: The central region of a cyclone must be a region of rarefaction, and, consequently, a region of refrigeration.
These currents appear as white lines on the surface of the wall; the cause probably being that the extreme rarefaction of the air by the heat of the body enables the rays of light to pass through them with less refraction than through the denser and more moist surrounding cold air.
"Yes," was the unanimous reply: and Mulrady added, "I'll carry the boy." The march eastward was forthwith resumed. They had a frightful height to climb yet to gain the topmost peaks. The rarefaction of the atmosphere produced that painful oppression known by the name of PUNA. Drops of blood stood on the gums and lips, and respiration became hurried and difficult.
The month of September was displaying its treasures; the atmosphere glowed above the grass and the pebbles; no cloud dimmed the blue of the sky, the purity of which in all parts, even close to the horizon, showed the extreme rarefaction of the air.
As the rarefaction took place, the water would of course rise in the tube; and the height to which it rose above the surface of the water in the bucket was made the measure of the amount of rarefaction.
It is marvelous to think, while looking upon their broad, level surfaces as smooth, apparently, as polished steel, though thirty thousand miles across that they are in reality vast circling currents of meteoritic particles or dust, through which run immense waves, condensation and rarefaction succeeding one another as in the undulations of sound.
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