THE POLAR AND TROPICAL ATMOSPHERE. 99 At 20° S. Mean Barometer 30 30. Mean Temperature 75°. 29.78. ⚫52. 20°. 55°. A very striking and remarkable difference still obtains. The barometer in the warmer region of the tropics, where the air is lighter, presents the higher barometer by absolutely half-an-inch, being quite the reverse of what it should be by our views of it. These speculative inquiries are not so useless or barren as might be supposed, for they lie at the root of all our opinions in respect to the general action of the barometer. They involve questions of importance, and from the total want of accordance between the barometer and thermometer, it would seem to be most unnecessary to make any correction for temperature in the barometric observations. The absolute reading is the true one, and more accordant to nature than the applied corrections. If we assume the atmosphere to be of one uniform height in every region of the globe, how are we to reconcile it with the different weights of the same bulk of air? Two columns of air having the same height, and being of the same absolute weight at the base, must have the same amount of heat in the columns, although the heat may be very differently distributed in the columns. The temperature at the base of the two columns may differ 120°, but a principle of compensation may exist in both; the warmer or tropical column may decrease rapidly in temperature as it ascends and leaves the warm and heated earth; the colder or polar column 561414 100 AIR IN EXPANDING DIVIDES ITS HEAT. may decrease but little, and at no very inconsiderable height may become relatively, if not absolutely, warmer than at the base. The following statement appears in the most scientific works : "The air in the higher regions of the atmosphere is much cooler than below; as it ascends from the earth it expands, and in expanding its capacity for heat is increased, it therefore cools itself as well as the surrounding bodies. The Andes, placed almost under the equator, rise in the midst of burning sands: about the middle height is a pleasant and mild climate; but the summits are covered with eternal snows; and these ranges of temperature are always distinct. The hot winds from below, if they ascend, become cooled in consequence of expansion; and the cold air, if by any force of the blast it is driven downward, is condensed and rendered warmer as it descends." Brande's Manual, page 65. This is to a certain degree true, for if a cubic foot of air, at the temperature of 80 degrees, becomes two cubic feet by diminished pressure at about 18,000 feet altitude, of course, as the air is divided or expanded into two volumes, its heat is likewise divided into two parts or half; and the temperature of the double mass is only half that of the original volume. Its temperature is divided. There is no chemical law or capacity for heat involved in it—it is a plain, obvious, and mechanical division of heat, accompanying the mechanical expansion of the air from diminished pressure. And THE LONG-CONTINUED TWILIGHT AT THE POLES. 101 this important principle will produce widely different effects in the tropics and polar regions, and tend materially and quickly to equalize the temperature of the upper portions of the atmosphere. But there is likewise another consideration which we have adverted to and adopted in the extract from Professor Faraday on the magnetic condition of the atmosphere (vide page 36, etc., in our 2nd Chapter). If we are authorized to receive such statements, then the upper regions of the polar atmosphere most beautifully illustrate his theory, and the opinions I am desirous of advocating receive the sanction of his authority. From the general concurrence of the facts and opinions, there is every reason to assume that the upper and loftier regions of the atmosphere within the polar circles, are absolutely warmer than the lower, and that the order of the position of the strata of the atmosphere is reversed at the poles to that of the tropics. There is almost a continuous play of twilight around the pole, for the sun is never long below the twilight degree of declination-the sun is only for a very short time more than 23° below the horizon of the pole. But in the tropics the sun descends every twenty-four hours to 90° below the horizon. And when we add the amazing refractive power of the polar atmosphere, it becomes a probable opinion, that its upper regions, being almost continually illumined, must partake in some degree of the heating power, although slight, but the continued action must produce some effect. And if light has a magnetic power, here then is its seat. The electric, 102 REVERSE ATMOSPHERIC STATES. chemical, and thermal condition of the upper regions of the polar atmosphere is everywhere as different from that of the tropics, as much as the base of the atmos phere of the two regions differ. The conditions are reversed. But this, some will say, is absurd, or at least hypothetical; but we have an analogy in the waters of the ocean. In the tropics the warm and tepid waters float upon the surface; but in the polar regions its surface is cold and frozen, or at any rate of icy temperature, but the bottom water is warmer and the abode of life. It is protected by the column above; it has no evaporation, no radiation; it is free from atmospheric influence; and by the special law by which Infinite wisdom has fixed the point of the greatest density of water at 40° Fahrenheit, the heaviest water sinks to the bottom, so that the warm water of 40° Fahrenheit reposes on the bed of the ocean, in every region of the globe alike. The depths of the ocean are not below that temperature at the pole itself-although mountains of ice float upon its surface. We are not warranted in relying too much on analogies, but we are warranted in believing that the same power that reverses the order of position of the waters of the ocean, as regards its temperature, may likewise, by any mode or modes unknown to us, reverse the order of the aërial ocean; and warmer air may occupy the loftier realms of the polar skies than in the tropics, or else the height of the columns cannot be equal, with such amazing difference of temperature. PROFESSOR DANIELL ON THE POLAR Atmosphere. 103 Professor Daniell, in his able and elegant work on Meteorology, says: "The elasticity of the air, as measured by the barometer, remaining the same, its specific gravity is very much greater at the poles than the equator; hence the atmospheric column must be proportionally shorter at the poles, and this heavier fluid must press upon and displace the lighter, and produce a current from the poles to the equator. "But the difference of gravity becomes less as we ascend, and at a certain point is neutralized; while, on the other hand, the elasticity, which is equal at the surface, varies with the height, and the barometer stands higher at equal elevations in the equatorial than in the polar column. This disproportion increases with the elevation, and, at some definite height, must more than compensate the unequal density of the lower strata, and occasions a counter flux from the equator to the poles, so that, at a height of 20,000 feet, the barometer at the poles would be 11 inches, but 13 inches at the equator, the temperature being 82° below zero at the poles, while the temperature at the same height in the tropics would be 12° above zero." It is neither pleasant nor reputable to differ from so great an authority, and the reader must adopt the opinion most consonant to the general facts. There are some other points which deserve a notice. The mean barometer in the polar regions is 29.89 inches. This weight gives it no preponderance or power over that of the more temperate climates; for, although the air is extremely cold and dense, yet this |