Part 60
"When the effects of the earth's rotation are taken into consideration, our reasoning must be as follows: the air over any part of the earth's surface, when apparently at rest, or calm, will have the same rotatory velocity as that part; but if a quantity of air in the northern hemisphere receive an impulse in the direction of the meridian, either northward or southward, its rotatory velocity will be greater in the former case, and less in the latter, than that of the air into which it moves; consequently, if it move northward, it will have a greater velocity eastward than the air, or surface of the earth over which it moves, and will therefore become a south-west wind, or a wind between the south and west; and, _vice versa_, if it move southward, it becomes a north-east wind. From similar considerations it will appear, that in the southern hemisphere the winds will be north-west and south-east respectively.
"The trade-winds may therefore be explained thus: The two general masses of air proceeding from both hemispheres towards the equator, as they advance are constantly deflected more and more towards the east, by reason of the earth's rotation; that from the southern hemisphere, originally a south wind, is made to veer more and more towards the east; in like manner, that from the northern hemisphere is made to change its directions from the north towards the east: these two masses meeting near the equator, their velocities south and north destroy each other, and they proceed afterwards with their common velocity from east to west round the torrid zone, excepting the irregularities produced by the continents. The equator is not in reality the place of concourse, but the northern parallel of four degrees; because the centre of heat is thereabouts, the sun being longer on the north side of the equator, than on the south side. Moreover, when the sun is near one of the tropics, the centre of heat upon the earth's surface is then nearer that tropic than usual, and therefore the winds about the tropic are more nearly east at that time, and those about the other tropic more nearly north and south.
"If all the terrestrial globe were covered with water, or if the variations of the earth's surface in heat were regular and constant, so that the heat was the same in every part of the same parallel of latitude, the winds would then be very nearly regular also: but this is not the case; for we find the irregularities of heat, arising from the interspersion of land and sea, are such, that though all the parts of the atmosphere in some measure conspire to produce regular winds about the torrid zone, yet very striking irregularities are often found to take place. A remarkable instance we have in monsoons, which are winds that in the Indian ocean, &c. blow for six months together one way, and the next six months the contrary way: these, with sea and land breezes, do not seem easily to be accounted for on any other principle than that of rarefaction.
"Perhaps some persons may be led to suppose, that the winds in the northern temperate zone should be between the north and east towards the poles, and between the south and west nearer the equator, almost as regular as the trade-winds: but when the change of seasons, the different capacities of land and water for heat, the interference and opposition of the two general currents, be considered, it might be concluded almost next to impossible that the winds in the temperate zone should exhibit any thing like regularity: however, notwithstanding this, observations sufficiently evince, that the winds therein are, for the most part, in the direction of one of the general currents; namely, somewhere between the south and west, or more commonly between the north and east; and that winds in other directions happen only as accidental varieties, chiefly in unsettled weather.
"We may have frequently taken notice, that several winds, particularly stormy ones, are attended with a cloudy sky; to this it may be added, that we have more winds than usually occur in rather less latitudes, where the atmosphere is generally more serene: these considered, make it exceedingly probable, that the aqueous vapours which are sustained by the air, from whence come clouds and rains, may be one great cause of irregular winds. It has been determined, from very accurate experiments, that one inch of water when evaporated will fill more than 2000 inches of space: hence it appears that the water which falls in drops of rain, &c. occupied more than 2000 times the space when it floated in the atmosphere in vapours; the condensation thereof must therefore occasion vacua of such a nature, as will cause winds of different kinds and degrees, according to the deficiency which is to be supplied.
"The economy of winds, an illustration of which has been here attempted, is admirably adapted to the various purposes of nature, and to the general intercourse of mankind:--if the earth had been fixed, and the sun had revolved about it, the air over the torrid zone, and particularly about the equator, would have been almost always stagnant; and in the other zones the winds would have had little variation either in direction or strength; in this case navigation would have been greatly impeded, and a communication between the two hemispheres by sea, rendered impracticable. On the present system of things, however, the irregularity of winds is of the happiest consequence, by being subservient to navigation: and a general circulation of air constantly takes place between the eastern and western hemispheres, as well as between the polar and equatorial regions; by reason of which, that diffusion and intermixture of the different aërial fluids, so necessary for the life, health, and prosperity of the animal and vegetable kingdoms, is accomplished:--such is the transcendent wisdom and providential care of the beneficent Father of all!"
The following interesting description of the SOUTH-WEST MONSOON IN INDIA, is taken from Elphinstone's Account of Cabul.--The most remarkable rainy season, is that called in India the South-west Monsoon. It extends from Africa to the Malay Peninsula, and deluges all the intermediate countries within certain lines of latitude, for four months in the year. In the south of India, this monsoon commences about the beginning of June, but it gets later as we advance towards the north. Its approach is announced by vast masses of clouds that rise from the Indian ocean, and advance towards the north-east, gathering and thickening as they approach the land. After some threatening days, the sky assumes a troubled appearance in the evenings, and the monsoon in general sets in during the night. It is attended with such a thunder-storm as can scarcely be imagined by those who have only seen that phenomenon in a temperate climate. It generally begins with violent blasts of wind, which are succeeded by floods of rain. For some hours, lightning is seen almost without intermission; sometimes it only illuminates the sky, and shews the clouds near the horizon; at others, it discovers the distant hills, and again leaves all in darkness; when in an instant it re-appears in vivid and successive flashes, and exhibits the nearest objects in all the brightness of day. During all this time the distant thunder never ceases to roll, and is only silenced by some nearer peal, which bursts on the ear with such a sudden and tremendous crash, as can scarcely fail to strike the most insensible heart with awe. At length the thunder ceases, and nothing is heard but the continued pouring of the rain, and the rushing of the rising streams. The next day presents a gloomy spectacle: the rain still descends in torrents, and scarcely allows a view of the blackened fields; the rivers are swollen and discoloured, and sweep down along with them the hedges, the huts, and the remains of the cultivation, which was carried on during the dry season, into their beds.
This lasts for some days, after which the sky clears, and discovers the face of nature, changed as if by enchantment. Before the storm, the fields were parched up, and, except in the beds of the rivers, scarce a blade of vegetation was to be seen; the clearness of the sky was not interrupted by a single cloud, but the atmosphere was loaded with dust, which was sufficient to render distant objects dim, as in a mist, and to make the sun appear dull and discoloured, till he attained a considerable elevation: a parching wind blew like a blast from a furnace, and heated wood, iron, and every other solid material, even in the shade; and immediately before the monsoon, this wind had been succeeded by still more sultry calms. But when the first violence of the storm is over, the whole earth is covered with a sudden but luxuriant verdure: the rivers are full and tranquil, the air is pure and delicious; and the sky is varied, and embellished with clouds. The effect of the change is visible on all the animal creation, and can only be imagined in Europe, by supposing the depth of a dreary winter to start at once into all the freshness and brilliancy of spring. From this time the rain falls at intervals for about a month, when it comes on again with great violence, and in July the rains are at their height; during the third month, they rather diminish, but are still heavy; and in September they gradually abate, and are often entirely suspended till near the end of the month, when they depart amidst thunders and tempests, as they came.
The following Table, which gives some particulars respecting the VELOCITY OF THE WIND, was calculated by Mr. John Smeaton, the celebrated engineer, and is founded on a correct series of practical observations:--
Miles Feet per hour. per second. 1 1.47 2 2.93 } Light airs. 3 4.40 } 4 5.87 } Breeze. 5 7.33 } 10 14.67 } Brisk gale. 15 22. } 20 29.34 } Fresh gale. 25 36.67 } 30 40.01 } Strong gale. 35 51.34 } 40 58.68 } Hard gale. 45 66.01 } 50 73.35 } Storm. 60 88.02 } 80 117.36 } Hurricane, tearing 100 146.70 } up trees, &c.
We conclude this chapter with a poetical enumeration of the benefits arising from the wind:--
"Of what important use to human kind, To what great ends subservient, is the wind! Where'er the aërial active vapour flies, It drives the clouds, and ventilates the skies; Sweeps from the earth infection's noxious train, And swells to wholesome rage the sluggish main. For should the sea unagitated stand, Death, with huge strides, would desolate the land; The scorching sun, with unpropitious beam, Would give to grief an everlasting theme; And baneful vapours, lurking in the veins, Would fiercely burn with unabating pains. Nor thus alone air purities the seas, O'er torrid climes it pours the healthful breeze: Climes where the sun direct flings scorching day, Feel cooling air his sultry rage allay; Unceasing goodness, with unceasing skill, Educing certain good from seeming ill. His guardian care extends o'er ev'ry shore, And blends his favours with what men deplore; The sable nations hence, and burning skies, See luscious fruits in varying beauty rise; Spontaneous Nature laugh at culture's toil, And rich luxuriance bless the grateful soil."
CHAP. L.
CURIOSITIES RESPECTING SHOWERS, STORMS, &c.
_Surprising Showers of Hail--Singular Effects of a Storm--The Mirage--Sand Floods--Showers of Gossamers--Winter in Russia._
Ye vapours, hail, and snow, Praise ye th' Almighty Lord, And stormy winds that blow To execute his word. _Watts._
Then from aërial treasures downwards pours Sheets of unsully'd snow in lucid show'rs; Flake after flake, thro' air thick wav'ring flies 'Till one vast shining waste all nature lies. Then the proud hills a virgin whiteness shed, A dazzling brightness glitters from the mead; The hoary trees reflect a silver show, And groves beneath the lovely burden low. _Broome._
SURPRISING SHOWERS OF HAIL.
Natural historians record various instances of surprising showers of hail, in which the hailstones were of extraordinary magnitude. Mezeray, speaking of the war of Lewis XII. in Italy, in 1510, relates, that there was for some time a horrible darkness, thicker than that of night; after which the clouds broke into thunder and lightning, and there fell a shower of hailstones, or rather (as he calls them) pebble-stones, which destroyed all the fish, birds, and beasts, of the country. It was attended with a strong smell of sulphur; and the stones were of a bluish colour, some of them weighing 100lb.--_Hist. de France_, tom. ii. p. 339.
At Lisle, in Flanders, in 1686, hailstones fell of a very large size; some of which contained in the middle, a dark brown matter, which, thrown on the fire, gave a very great report.--_Phil. Trans._ No. 203.
Dr. Halley and others relate, that in Cheshire, Lancashire, &c. April 29, 1697, a thick black cloud, coming from Carnarvonshire, disposed the vapours to congeal in such a manner, that for about the breadth of two miles, which was the limit of the cloud, in its progress for sixty miles it did inconceivable damage; not only killing all sorts of fowls and other small animals, but splitting trees, knocking down horses and men, and even ploughing up the earth, so that the hailstones buried themselves underground an inch or an inch and a half deep. The hailstones, many of which weighed five ounces, and some half a pound, being five or six inches in circumference, were of various figures; some round, others half round; some smooth, others embossed and crenated; the icy substance of them was very transparent and hard, but there was a snowy kernel in the centre.
In Hertfordshire, May 4, 1697, after a severe storm of thunder and lightning, a shower of hail succeeded, which far exceeded the former: some persons were killed by it, their bodies beat all black and blue; vast oaks were split, and fields of rye cut down as with a scythe. The stones measured from ten to fourteen inches round. Their figures were various, some oval, some flat, &c.--_Phil. Trans._ No. 229.
The following account of the SINGULAR EFFECTS OF A STORM, was communicated to the Dublin Philosophical Society, by the secretary:--
"Mrs. Close gave Mr. Molyneux the following account of the effects of thunder and lightning on her house at New Forge, in the county of Down, in Ireland, on August 9, 1707: She observed, that the whole day was close, hot, and sultry, with little or no wind stirring till towards the evening, when there was a small breeze, with some mizzling rain, which lasted about an hour; that as the air darkened after sunset, she saw several faint flashes of lightning, and heard some thunder-claps at a distance; that between ten and eleven o'clock, both were very violent and terrible, and so increased, and came on more frequently until a little before twelve o'clock; that one flash of lightning and clap of thunder came both at the same time, louder and more dreadful than the rest, which, as she thought, shook and inflamed the whole house; and being sensible at that instant of a violent strong sulphureous smell in her chamber, and feeling a thick gross dust falling on her hands and face as she lay in bed, she concluded that part of her house was thrown down by the thunder, or set on fire by the lightning; that, arising in this fright, she called up her family, and candles being lighted, she found her bedchamber, and the kitchen beneath it, full of smoke and dust, and the looking-glass in her chamber was broken.
"The next day she found, that part of the cornice of the chimney, which stood without the gabel-end of the house where her chamber was, had been struck off; that part of the coping of the splay of the gable-end itself was broken down, and twelve or sixteen of the shingles on the adjoining roof were raised or ruffled, but none shattered or carried away; that a part of the ceiling in her chamber beneath those shingles was forced down, and part of the plaster and pinning stones of the adjoining wall was also broken off and loosened, the whole breach being sixteen or twenty inches broad; that at this place there was left on the wall a smutted scar or trace, as if blacked by the smoke of a candle, which pointed downwards towards another place on the same wall, where a like breach was made, partly behind the place of the looking-glass; that the boards on the back of a large hair-trunk, full of linen, standing beneath the looking-glass, were forced in, and splintered as if by the blow of a smith's sledge; that two-thirds of the linen within this trunk were pierced or cut through, the cut appearing of a quadrangular figure, and between two or three inches over; that one end of the trunk was forced out, as the back was driven in; that at about two feet distance from the end of the trunk, where the floor and the side-wall of the house joined, there was a breach made in the plaster, where a small chink or crevice was to be seen between the sideboard of the floor and the wall, so wide that a man could thrust his lingers down; and that just beneath this, in the kitchen, the ceiling was forced down, and some of the plaster of the wall broken off; that exactly under this there stood a large tub or vessel of wood, inclosed with a crib of brick and lime, which was broken and splintered all to pieces, and most of the brick and lime work of it scattered about the kitchen.
"The looking-glass was broken with such violence, that there was not a piece of it to be found of the size of half-a-crown, and several pieces of it were sticking like hail-shot in the chamber door, which was of oak, and on the other side of the room; several of the edges and corners of some of the pieces of the broken glass were tinged of a light flame colour, as if heated in the fire; the curtains of the bed were cut in several places, supposed to be done by the pieces of the glass. Several pieces of muslin and wearing linen, left on the large hair-trunk, were thrown about the room, no way singed or scorched, and yet the hair on the back of the trunk, where the breach was made, was singed; the uppermost part of the linen within the trunk was not touched, and the lowermost parcel, consisting of more than 350 ply of linen, was pierced through, of which none was anywise smutted, except the uppermost ply of a tablecloth, that lay over all the rest; there was a yellow tinge or stain perceivable on some part of the damaged linen, and the whole smelt strongly of sulphur; the glass of two windows in the bed-chamber above, and two windows in the kitchen below, were so shattered, that there was scarcely one whole frame left, in many of them. The pewter, brass, and iron furniture in the kitchen, were thrown down, and scattered about; particularly, a large girdle, about twenty pounds weight, that hung upon an iron hook near the ceiling, was found lying on the floor. A cat was found dead next morning in the kitchen, with her legs extended in a moving posture, with no other sign of being hurt, than that the fur was singed a little about the rump.
"It was further remarkable, that the wall, both above and below a little window in the same gable-end, was so shattered, that the light could be seen through the crevices in the wall, and upon a large stone on the outside of the wall; beneath this window was a mark, as if made by the stroke of a smith's, sledge, and a splinter of the stone was broken off, of some pounds weight. I was further informed, that from the time of the great thunder-clap, both the thunder and lightning diminished gradually, so that in an hour's time all was still and quiet again."
We proceed to give an account of THE MIRAGE.--From Belzoni's Narrative.
"This phenomenon is often described by travellers, who assert having been deceived by it, as at a distance it appears to them like water. This is certainly the fact, and I must confess that I have been deceived myself, even after I was aware of it. The perfect resemblance to water, and the strong desire for this element, made me conclude, in spite of all my caution not to be deceived, that it was really water I saw. It generally appears like a still lake, so unmoved by the wind, that every thing above it is to be seen most distinctly reflected, which is the principal cause of the deception. If the wind agitate any of the plants that rise above the horizon of the mirage, the motion is seen perfectly at a great distance. If the traveller stands elevated much above the mirage, the water seems less united and less deep, for, as the eyes look down upon it, there is not thickness enough in the vapour of the surface of the ground to conceal the earth from the sight; but if the traveller be on a level with the horizon of the mirage, he cannot see through it, so that it appears to him clear water. By putting my head first to the ground, and then mounting a camel, the height of which from the ground might have been ten feet at the most, I found a great difference in the appearance of the mirage. On approaching it, it becomes thinner, and appears as if agitated by the wind, like a field of ripe corn. It gradually vanishes as the traveller approaches, and at last entirely disappears when he is on the spot."