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They are owing to currents of air running upon each other in horizontal strata, differing in their electric state, in temperature and moisture, as well as in velocity and direction.
When north and south winds blow alternately, the wind at any place will veer in one uniform direction through every point of the compass, provided the one begins before the other has ceased. In the northern hemisphere a north wind sets out with a smaller degree of rotatory motion than the places have at which it successively arrives, consequently it passes through all the points of the compass from N. to N.E. and E. A current from the south, on the contrary, sets out with a greater rotatory velocity than the places have at which it successively arrives, so by the rotation of the earth it is deflected from S. to S.W. and W. Now, if the vane at any place should have veered from the N. through N.E. to E., and a south wind should spring up, it would combine its motion with the former and cause the vane to turn successively from the E. to S.E. and S. But by the earth's rotation this south wind will veer to the S.W. and W., and, if a north wind should now arise, it would combine its motion with that of the west, and cause it to veer to the N.W. and N. Thus two alternations of north and south wind will cause the vane at any place to go completely round the compass, from N. to E., S., W., and N. again. At the Royal Observatory at Greenwich the wind accomplishes five circuits in that direction in the course of a year. When circumstances combine to produce alternate north and south winds in the southern hemisphere, the gyration is in the contrary direction. Although the general tendency of the wind may be rotatory, and is so in many instances, at least for part of the year, yet it is so often counteracted by local circumstances, that the winds are in general very irregular, every disturbance in atmospheric equilibrium from heat or any other cause producing a corresponding wind. The most prevalent winds in Europe are the N.E. and S.W.; the former arises from the north polar current, and the latter from causes already mentioned. The law of the wind's rotation was first described by Dr. Dalton, but has been developed by Professor Dove, of Berlin.
Hurricanes are those storms of wind in which the portion of the atmosphere that forms them revolves in a horizontal circuit round a vertical or somewhat inclined axis of rotation, while the axis itself, and consequently the whole storm, is carried forward
along the surface of the globe, so that the direction in which the storm is advancing is quite different from the direction in which the rotatory current may be blowing at any point. In the West Indies, where hurricanes are frequent and destructive, they generally originate in the tropical regions near the inner boundary of the trade-winds, and are caused by the vertical ascent of a column of rarefied air, whose place is supplied by a rush of wind from the surrounding regions, set into gyration by the rotation of the earth. By far the greater number of Atlantic hurricanes have begun eastward of the lesser Antilles or Caribbean Islands.
In every case the axis of the storm moves in an elliptical or parabolic curve, having its vertex in or near the tropic of Cancer, which marks the external limit of the trade-winds north of the equator. As the motion before it reaches the tropic is in a straight line from S.E. to N.W., and after it has passed it from S.W. to N.E., the bend of the curve is turned towards Florida and the Carolinas. In the southern hemisphere the body of the storm moves in exactly the opposite direction. The hurricanes which originate south of the equator, and whose initial path is from N.E. to S.W., bend round at the tropic of Capricorn, and then move from N.W. to S.E.
The extent and velocity of these storms are great; for instance, the hurricane that took place on the 12th of August, 1830, was traced from eastward of the Caribbee Islands, along the Gulf Stream, to the bank of Newfoundland, a distance of more than 3000 miles, which it passed over in six days. Although the hurricane of the 1st of September, 1821, was not so extensive, its velocity was greater, as it moved at the rate of 30 miles an hour: small storms are generally more rapid than those of greater dimensions.
The action of these storms seems to be at first confined to the stratum of air nearest the earth, and then they seldom appear to be more than a mile high, though sometimes they are raised higher; or even divided by a mountain into two separate storms, each of which continues its new path and gyrations with increased violence. This occurred in the gale of the 25th of December, 1821, in the Mediterranean, when the Spanish mountains and the Maritime Alps became new centres of motion.
By the friction of the earth the axis of the storm bends a little forward. This causes a continual intermixture of the lower and
warmer strata of air with those that are higher and colder, producing torrents of rain and violent electric explosions.
The breadth of the whirlwind is greatly augmented when the path of the storm changes on crossing the tropic. The vortex of a storm has covered an extent of the surface of the globe 500 miles in diameter.
The revolving motion accounts for the sudden and violent changes observed during hurricanes. In consequence of the rotation of the air, the wind blows in opposite directions on each side of the axis of the storm, and the violence of the blast increases from the circumference towards the centre of gyration, but in the centre itself the air is in repose: hence, when the body of the storm passes over a place, the wind begins to blow moderately, and increases to a hurricane as the centre of the whirlwind approaches; then, in a moment, a dead and awful calm succeeds, suddenly followed by a renewal of the storm in all its violence, but now blowing in a direction diametrically opposite to its former course. This happened at the Island of St. Thomas on the 2nd of August, 1837, where the hurricane increased in violence till half-past seven in the morning, when perfect stillness took place for forty minutes, after which the storm recommenced in a contrary direction.
The sudden fall of the mercury in the barometer in the regions habitually visited by hurricanes is a certain indication of á coming tempest. In consequence of the centrifugal force of these rotatory storms the air becomes rarefied, and, as the atmosphere is disturbed to some distance beyond the actual circle of gyration or limits of the storm, the barometer often sinks some hours before its arrival, from the original cause of the rotatory disturbance. It continues sinking under the first half of the hurricane, is at a maximum sometimes of two inches in the centre of gyration, and again rises during the passage of the latter half, though it does not attain its greatest height till the storm is over. The diminution of atmospheric pressure is greater and extends over a wider area in the temperate zones than in the torrid, on account of the sudden expansion of the circle of rotation when the gale crosses the tropic.
As the fall of the barometer gives warning of the approach of a hurricane, so the laws of the storm's motion afford the seaman knowledge to guide him in avoiding it. In the northern
temperate zone, if the gale begins from the S.E. and veers by S. to W., the ship should steer to the S.E.; but, if the gale begins from the N.E., and changes through N. to N.W., the vessel should go to the N.W. In the northern part of the torrid zone, if the storm begin from the N.E., and veer through E. to S.E., the ship should steer to the N.E.; but, if it begin from the N.W., and veer by W. to S.W., the ship should steer to the S.W., because she is in the south-western side of the storm. Since the laws of storms are reversed in the southern hemisphere, the rules for steering vessels are necessarily reversed also. A heavy swell is peculiarly characteristic of these storms. In the open sea the swell often extends many leagues beyond the range of the gale which produced it.
Waterspouts are occasioned by small whirlwinds, which always have their origin at a great distance from that part of the sea from which the spout begins to rise, where it is generally calm. The whirl is produced by two currents of air, which, running in opposite directions, compress one another by their impetus, so that they rise in spiral eddies to the clouds. They move slowly along the surface of the sea, sometimes in vertical, and sometimes in twisted spirals, putting the sea into violent agitation as they pass, and carrying the water aloft by the force of gyration. Occasionally the eddies begin in the clouds and dip down to the sea.
Sound - Propagation of Sound illustrated by a Field of Standing Corn Nature of Waves - Propagation of Sound through the AtmosphereIntensity Noises - A Musical Sound Quality - Pitch Extent of Human Hearing-Velocity of Sound in Air, Water, and Solids— Causes of the Obstruction of Sound-Law of its Intensity - Reflection Echoes-Thunder- Refraction of Sound Interference
of Sound of Sounds.
ONE of the most important uses of the atmosphere is the conveyance of sound. Without the air, deathlike silence would prevail through nature, for in common with all substances it has a tendency to impart vibrations to bodies 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 in every direction, and produce the sensation of sound upon the auditory nerves. A bell rung under the exhausted receiver of an air-pump is inaudible, which shows that the atmosphere is really the medium of sound. In the small undulations of deep water in a calm, the vibrations of the liquid particles are made in the vertical plane, that is, up and down, or at right angles to the direction of the transmission of the waves. But the vibrations of the particles of air which produce sound differ from these, being performed in the same direction in which the waves of sound travel. The propagatio of sound has been illustrated by a field of corn agitated by the wind. However irregular the motion of the corn may seem on a superficial view, it will be found, if the velocity 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