But while the atmosphere may be considered generally as an aerial zone of the earth, the companion of the massy spheroid in its annual revolution round the sun, and rotating with it upon its axis, it has independent movements which present very complex phenomena, however clear the causes which put them in operation. The particles of air are constantly suffering displacement, and it is easy to conceive of various circumstances disturbing the dilatable and elastic fluid in which we live. A body in movement will communicate its motion to the adjoining particles, which may be sensibly propagated by them to a considerable distance; but this cause operates so slightly in the production of atmospheric currents that it might be entirely overlooked. It will be sufficient to state that some of the vast oceanic streams are supposed to produce a corresponding flow in the air. The varying attractions of the sun, moon, and planets on the atmosphere, will occasion tides in it analogous to those of the ocean, or an alteration in the heights of vertical columns of air, winds and currents arising from the resulting inequalities of horizontal pressure; but La Place has proved the action of this cause to be scarcely appreciable. The atmospheric agitations of which we are sensible, both the more violent and gentle, appear to proceed either from a change in the temperature of a portion of the air, or from a change in the quantity of water which it holds in a state of vapor. In both these cases a temporary destruction of the equilibrium subsisting between different parts of the atmosphere is produced, and its particles are set in motion to restore the balance. The effect of heat upon a volume of air is to rarefy and expand, to increase its bulk and diminish its density. When any portion, therefore, of the earth’s surface is more heated than the surrounding districts, the air there ascends and flows over the adjoining cooler and denser strata, causing an upper outward current, while the colder and denser fluid rushes toward the spot where the balance has been lost by expansion, and a lower inward current is produced. An easy experiment will illustrate this interchange. In a room warmed by a good fire, if a candle be held at the crevice between the door and the floor, an inward current will be observed from the exterior colder air, but near the ceiling, by the same means, an outward flow will be detected. In the other condition an addition of vapor to the atmosphere gives rise to a wind blowing on all sides away from the district of evaporation, while an abstraction of it by showers creates a partial vacuum, toward which the air rushes from all points of the compass. The diversity of the winds in power is principally owing to the different degrees of vigor with which these causes act.