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THE TIDES OF THE SEA AND THE TIDES OF THE AIR      189

And now as to the tides of the atmosphere.  While they may not be so complex in their motion as the tides of the sea, because the ambient air is not encumbered with solid and obstructing continents to its movements, they are none the less grand and imposing when viewed in their positive action, and their economy in the great workshop of the universe of matter.  The reason that we have no air-tide theories in the books is, because it is not long since men have learned how to get into the ocean above us, and even since that art has been acquired, very few have availed themselves of its uses.  But it is a law of natural selection, that as mankind grows wiser, and more refined, so will also mankind aspire to occupy more room in space, especially that of the heavens above, and then we shall have more science of meteorology, and a better explanation of things directly connected with that science.

That there are atmospheric tides is no longer a question, but how they move, and what governs them, is a question, and one we are now going to briefly consider.  The motions of the air, in what we shall consider as the great atmospheric tides, is caused by the same power, and governed by the same laws, as the motion of the sea.  It flows round our globe from west to east, faster than the surface of its more solid matter, and this motion is caused by centrifugal force springing out of the rotary motion of our planet, agreeably to the law of mechanical forces.  I can perceive no difference between celestial, or natural, mechanical forces, and those of artificial, or human contrivances.  While this great atmospheric wave from west to east has no continents and shore-lines to give it the numerous variations of the ocean tide, it has nevertheless complex motions caused by the difference of temperature, to which the chambers of heaven are more healthfully and more systematically ventilated than the best ventilated mansion that was ever constructed by mortal man.  And this is an established scientific fact of every day's experience.

To a letter I wrote to Prof. Henry in 1849, on the subject of the law of gravitation, and the practicability of crossing the Atlantic ocean in from two to three days with a balloon, his answer, as to the latter proposition, was: "I have no doubt that there are great currents in the upper regions of the atmosphere, and particularly the return-currents of the trade-winds, which should blow continually from southwest to northeast.  Should you conclude to make another aerial voyage, I should be pleased to suggest some observations."  These currents are the aerial tides, caused in this way: Around the equatorial belt of our globe we find the greatest amount of heat, as also the greatest amount of centrifugal force, and the combination of these two elements make it the great seething laboratory wherein are generated the phenomenal disturbances - no, not disturbances, but the sublime circulation of the ethereal fluid which we recognize in the gentle trade-winds, in the more animated equinoctial storms, in the roaring cyclone, and the fierce thunder-gust, distributing electricity and moisture to every part of the earth, ventilating all its departments of land and water, and giving vitality to all its beings.  The tropical trade-winds blowing in from the northeast on the north side of the equator, and from the southeast on its south side, apparently contradicting the general flow of atmosphere from west to east, are really normal expressions of its scientific condition.  Friction, heat, and convection drawing in the air below by undertow, then being heated as it rises, it is hurled outward and forward, that is to say, towards the poles and towards the east giving rise to the upper and the lower currents of the aerial tides.  The lower currents only were recognized in the old trade-wind; the system of the upper current was first pointed out by an obscure individual.

This great breathing apparatus of the equatorial lungs, in its truly organic inhalations and exhalations, produce the interesting phenomenon of a cloud belt round our planet of several hundred miles in breadth, presenting to our next-door neighbors, Mars and Venus, and to their inhabitants, if they have any, an appearance similar to that we behold on the planet Jupiter.

This great aerial wave sends off north and south the upper currents of its regenerated air, and these currents losing the centrifugal force acquired at the equator in the ration as they proceed to higher latitudes, until they reach the polar circles, where there is very little eastward motion on the surface of the earth by its axial rotation, and where the same conditions of calm exists as in the calm belt of the equator, but just the opposite of the equator in temperature, the air is forced back from the poles, because it cannot be packed there beyond a normal pressure, and because it also moves in obedience to the law of temperature and direction of least resistance, to wit: that cold air will towards a warmer point, as warm air will flow towards a colder point.  We have this law