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while that which is heated is carried along the higher strata to the poles, forming two counter currents in the direction of the meridian. But the rotatory velocity of the air, corresponding to its geographical position, decreases towards the poles; in approaching the equator, it must therefore revolve more slowly than the corresponding parts of the earth, and the bodies on the surface of the earth must strike against it with the excess of their velocity, and, by its reaction, they will meet with a resistance contrary to their motion of rotation: : so that the wind will appear, to a person supposing himself to be at rest, to blow in a contrary direction to the earth's rotation, or from east to west, which is the direction of the trade winds.
The equator does not exactly coincide with the line which separates the trade winds north and south of it; that line of separation depends upon the total difference of heat in the two hemispheres, arising from the unequal length of their summers, the distribution of land and water, and other causes. There are many proofs of the existence of a counter current above the trade winds. On the Peak of Teneriffe, the prevailing winds are from the west. The ashes of the volcano of St. Vincent's, in the year 1812, were carried to windward as far as the island of Barbadoes by the upper current. The captain of a Bristol ship declared
that, on that occasion, dust from St. Vincent's fell to the depth of five inches on the deck at the distance of 500 miles to the eastward; and light clouds have frequently been seen moving rapidly from west to east, at a very great height above the trade winds, which were sweeping along the surface of the ocean in a contrary direction.
WITHOUT the atmosphere, death-like silence would prevail through nature, for it, in common with all substances, has a tendency to impart vibrations to those in contact with it, therefore undulations received by the air, whether it be from a sudden impulse, such as an explosion, or the vibrations of a musical chord, are propagated equally in every direction, and produce the sensation of sound upon the auditory nerves. In the small undulations of deep water in a calm, the vibrations of the liquid particles are made in the vertical plane, that is, at right angles to the direction of the transmission of the waves; but the vibrations of the particles of air which produce sound differ, being performed in the same direction in which the waves of sound travel. The propagation of sound may be illustrated by a field of corn agitated by a gust of wind; for however irregular the motion of the corn may seem, on a superficial view, it will be
found, if the intensity of the wind be constant, that the waves are all precisely similar and equal, and that all are separated by equal intervals, and move in equal times.
A sudden blast depresses each ear equally and successively in the direction of the wind, but in consequence of the elasticity of the stalks and the force of the impulse, each ear not only rises again as soon as the pressure is removed, but bends back nearly as much in the contrary direction, and then continues to oscillate backwards and forwards in equal times like a pendulum, to a less and less extent, till the resistance of the air puts a stop to the motion. These vibrations are the same for every individual ear of corn; yet as their oscillations do not all commence at the same time, but successively, the ears will have a variety of positions at any one instant. Some of the advancing ears will meet others in their returning vibrations, and as the times of oscillation are equal for all, they will be crowded together at regular intervals; between these, there will occur equal spaces where the ears will be few, in consequence of being bent in opposite directions; and at other equal intervals they will be in their natural upright positions; so that over the whole field there will be a regular series of condensations and rarefactions among the ears of corn, separated by equal intervals where
they will be in their natural state of density. In consequence of these changes the field will be marked by an alternation of bright and dark bands. Thus the successive waves which fly over the corn with the speed of the wind are totally distinct from, and entirely independent of, the extent of the oscillations of each individual ear, though both take place in the same direction. The length of a wave is equal to the space between two ears precisely in the same state of motion, or which are moving similarly, and the time of the vibration of each ear is equal to that which elapses between the arrival of two successive waves at the same point. The only difference between the undulations of a corn-field and those of the air which produce sound is, that each ear of corn is set in motion by an external cause, and is uninfluenced by the motion of the rest, whereas in air, which is a compressible and elastic fluid, when one particle begins to oscillate, it communicates its vibrations to the surrounding particles, which transmit them to those adjacent, and so on continually. Hence, from the successive vibrations of the particles of air, the same regular condensations and rarefactions take place as in the field of corn, producing waves throughout the whole mass of air, though each molecule, like each individual ear of corn, never moves far from its
state of rest. The small waves of a liquid, and the undulations of the air, like waves in the corn, are evidently not real masses moving in the direction in which they are advancing, but merely outlines, motions, or forms rushing along, and comprehending all the particles of an undulating fluid, which are at once in a vibratory state. Or, in other words, an undulation is merely the continued transmission in one direction of particles bearing a relative position to one another. It is thus that an impulse given to any one point of the atmosphere is successively propagated in all directions, in waves diverging as from the centre of a sphere to greater and greater distances, but with decreasing intensity, in consequence of the increasing number of particles of inert matter which the force has to move; like the waves formed in still water by a falling stone, which are propagated circularly all around the centre of disturbance. These successive spherical waves are only the repercussions of the condensations and motions of the first particles to which the impulse was given.
The intensity of sound depends upon the violence and extent of the initial vibrations of air, but whatever they may be, each undulation, when once formed, can only be transmitted straight forwards, and never returns back again unless when reflected by an opposing obstacle. The