As compared with water, the air (the barometer indicating 30°, and the thermometer 55°) is 833 times lighter.
It is this weight of the atmosphere which counterbalances that of a column of mercury 29 inches in height, and a column of water 32 to 34 feet in height.
The old quaint notion of Nature's abhorring a vacuum was found to be practically only an assertion that the air had weight. The ordinary pump, commonly called the suction-pump, is constructed on this principle. The weight of the atmosphere at the level of the sea is found to be the same all over the world.
We find the atmosphere with another characteristic,–Elasticity.
However it may be compressed, air returns, on liberation, to its original volume, and while thus perfectly elastic it is also the most compressible of bodies. This elasticity arises from the repulsive force of its particles, and is always equal to the compressive force which it balances. A glass vessel full of air, placed under a receiver and then exhausted by the air-pump, will burst into atoms. Water, on the other hand, is almost the reverse. Twenty cubic inches, introduced into a cannon whose sides are three inches thick, cannot be compressed into nineteen inches without bursting it. This non-elastic property of water, with another, that of communicating, when under the action of any force, an equal pressure in all directions, led to the invention of the hydraulic press.
The elasticity of the air enables fishes to rise and sink in water, through the action of the air-bladder.
The sudden compression of air liberates its latent heat, and produces fire. On this principle the pneumatic tinder-box is constructed.
Brockhaus says that air has as yet been compressed only into one-eighth of its original bulk.
For every degree of heat between the freezing-point and the boiling-point, 32° and 212°, the expansion of air is about 1/490th part, so that any invention which seeks to use rarefied air as a motive power must employ a very intense degree of heat, enough to fuse many kinds of metals.
To the celebrated Mr. Boyle and to Henry Cavendish, both of Great Britain, we are indebted for most of what we know of this particular property of the air.
Density, or closeness, is another quality of the atmosphere. It has been found to be 770 times less than that of water, and 770 cubic inches of air weigh as much as a cubic inch of water. It is in direct ratio with its elasticity, and there are tables by which it may be determined at different altitudes. At the surface of the earth, this density is indicated as 1; at 2-1/2 miles, as 1/2; at 5 miles, as 1/4; and so on, the difference being in a geometrical progression.
As we proceed in the consideration of our general subject, we shall find, under the appropriate heads, that density is not without material influence on reflection and refraction, on transparency and the transmission of light, the presence or absence of moisture, and the amount of heat at the earth's surface,– -and we might add, on health, and the increase or diminution of the vital energies.
Temperature is another branch of our subject, and one involving a series of subordinate topics on which volumes have been written, and to which are still devoted the labors of the most learned men of our day. In this place, merely an out-line can be attempted.
Temperature is the degree of heat or cold in the particles of all bodies, which is perceptible by sensation, and is measurable by their expansion or contraction. It is the key to the theory of the winds, of rain, of aerial and oceanic currents, of vegetation and climate with all their multifarious and important differences. While the inclined position of the earth on its axis and its movement in its elliptical orbit influence the general amount of heat, it is rather to the consequences of these in detail that we are called when we speak of temperature. If the sun shone on a uniformly level surface, everywhere of the same conducting and radiating power, there would be but little difficulty in tracing the monotonous effects of temperature.
The reformer Luther, as eccentric as he was learned and sincere, is reported to have said, that, if he had been consulted at the Creation, he would have placed the sun directly over the centre of the world and kept it there, to give unchanging and uniform light and heat! It is certainly much better that he was not consulted. In that case, every parallel of latitude would have been isothermal, or of equal mean annual temperature. The seasons would have been invariable in character. Some portions of the earth would have been scorched to crispness, others locked up in never-changing ice.
Vegetation, instead of being universal, would have been confined to a narrow zone; and the whole human race would have been driven together into one limited habitable space, to interfere with, incommode, and destroy each other. The arrangement is best as it is.
We find very important modifications of temperature, occasioned not only by astronomical influences, but by local causes and geographical characteristics. For while, as a general rule, the nearer we approach the equator, the warmer we shall be, yet temperature is greatly affected by mountains, seas, currents of air or water, by radiation, by forests, and by vegetation. It is found, in fact, that the lines of temperature, (the happy conception of Humboldt,) when they are traced upon the map, are anything but true zones or circles.
The line of the greatest mean warmth is not coincident with the equator, but falls to the north of it. This line at 160° W. Long, from Greenwich is 4° below the geographical equator; at 80° it is about 6° north, sweeping along the coast of New Granada; at 20° it comes down and touches the equator; at 40° E. Long., it crosses the Red Sea about 16° north of the equator, and at 120° it falls at Borneo, several degrees below it;–and the points of the greatest heat, in this line, are in Abyssinia, nearer the tropic of Cancer than to the equator. On the other hand, the greatest mean cold points, according to the opinions of Humboldt, Sir David Brewster, and others, do not coincide, as would seem natural, with the geographical poles, but they are both to be found in the northern hemisphere, in Latitude 80°, 95°E. Long. and 100° W. Long. from Greenwich. The western is ascertained to be 4-1/2° colder than the eastern or Siberian. If this be the fact,–but it is not positively admitted,–an open sea at the pole may be considered as probable, on the ground of its having a higher mean temperature than is found at 80°. Kaemptz places one of these cold points at the north of Barrow's Straits,–the other near Cape Taimur, in Siberia. Burghaus, in his Atlas, transfers the American cold pole to 78° N. Lat. It is perhaps too early to determine rigorously the true temperature of these points.
A noticeable fact also is this,–that places in the same latitude rarely receive the same amount of heat. Quebec, in British America, and Drontheim, in Norway, enjoy about the same quantity, while the former is in 47° and the latter in 68° N. Lat. The mean winter temperature of Pekin, 39° 45' N. Lat., is 5° below the freezing-point; while at Naples, which is north of Pekin, it seldom, if ever, goes below it, and Paris, 500 miles farther north, has a mean winter temperature of 6° above the freezing-point. The city of New York, about 11° south of London, has a winter temperature of much greater severity. The mean temperature of the State of New York, as determined by a long series of observations, is 44° 31'.
The mean temperature of countries is found to be very stable, and but very small variations have been detected in modern times. But that there have been important climatic changes, since the Christian era, cannot be doubted, unless we doubt history. Not many centuries ago, it was a common thing for all the British rivers to freeze up during the winter, and to remain so for several months. If space permitted, an interesting statement could he made of the changes which have taken place in vegetation in Greenland, and throughout certain northern parts of Europe,–also in Palestine, Greece, and other southern countries,–while we know that the earth's inclination upon its axis has been unchanged.
Mrs. Somerville remarks, that, though the temperature of any one place may be subject to very great variations, yet it never differs from the mean state more than a few degrees.
Without this atmospheric covering of ours, it is considered that the temperature of the earth at its surface would be the same as that of the celestial spaces, supposed to be at least 76° below zero, or possibly, says Humboldt, 1400° below! Human life, without our atmosphere, could not exist for