The Rain Cloud Part 4

CHAPTER V.

METHOD OF MEASURING THE QUANt.i.tY OF RAIN THAT FALLS-THE RAIN GAUGE-METHODS OF OBSERVING FOR RAIN AND SNOW-EFFECTS OF ELEVATION ON THE QUANt.i.tY OF RAIN-DIFFERENCE BETWEEN THE TOP OF A TALL BUILDING AND THE SUMMIT OF A MOUNTAIN-SIZE OF DROPS OF RAIN-VELOCITY OF THEIR FALL-QUANt.i.tY OF RAIN IN DIFFERENT LAt.i.tUDES-EXTRAORDINARY FALLS OF RAIN-REMARKS ON THE RAIN OF THIS COUNTRY-INFLUENCE OF THE MOON-ABSENCE OF RAIN-REMARKABLE DROUGHT IN SOUTH AMERICA-ITS TERRIBLE EFFECTS AND CONSEQUENCES-ARTIFICIAL RAINS.

The quant.i.ty of rain which falls at different parts of the earth's surface is very variable; and for the purpose of measuring it instruments called _Rain-gauges_ have been contrived. The simplest form is a funnel three or four inches high, and having an area of one hundred square inches. This may be placed in the mouth of a large bottle, and, after each fall of rain, the quant.i.ty may be measured by a gla.s.s jar divided into inches and parts. This simple gauge being placed on the ground in an open spot, will evidently represent a portion of the ground, and will show the depth of rain which would cover it at and about that spot, supposing the ground to be horizontal, and that the water could neither flow off nor sink into the soil. Thus, by taking notice of the quant.i.ty of rain which falls day by day, and year by year, and taking the average of many years, we get the mean annual quant.i.ty of rain for the particular spot in question. By an extension of these observations, it is evident that the mean annual fall of rain may be known for a district or a kingdom.

A more convenient form of rain-gauge than the one just noticed, is made by placing the funnel at the top of a bra.s.s or copper cylinder, connected with which at the lower point, is a gla.s.s tube with a scale, measuring inches and tenths of an inch. The water stands at the same height in the gla.s.s tube as it does in the cylinder, and being visible in the tube the height can be immediately read on the scale. The cylinder and the tube are so constructed, that the sum of the areas of their sections is a given part, such as a tenth of the area of the mouth of the funnel; so that each inch of water in the tube is equal to the tenth of an inch of water which enters the mouth of the funnel. A stop-c.o.c.k is added for drawing off the water from the cylinder after each observation is noted down.

Some rain-gauges are constructed for showing the quant.i.ty of rain which falls from each of the four princ.i.p.al quarters. Others are made so as to register, themselves, the quant.i.ty of rain fallen. One of this kind, by Mr. Crosley, consists of a funnel through which the rain pa.s.ses to a vibrating trough; when, after a sufficient quant.i.ty has fallen into its higher side, it sinks down and discharges the rain which escapes by a tube. The vibrating action of this trough moves a train of wheel-work and indices, which register upon a dial plate the quant.i.ty of rain fallen.

Whatever form of rain-gauge is adopted, it must be placed in an exposed situation, at a distance from all buildings, and trees, and other objects likely to interfere with the free descent of rain into the funnel. It is usual, in rainy weather, to observe the quant.i.ty of water in the gauge every morning; but this does not seem to be often enough, considering how freely water evaporates in an exposed situation. An error may also arise from some of the water adhering to the sides of the vessel, unless an allowance is made for the quant.i.ty thus lost by a contrivance such as the following:-Let a sponge be made damp, yet so that no water can be squeezed from it, and with this collect all the water which adheres to the funnel and cylinder, after as much as possible has been drawn off; then, if the sponge be squeezed, and the water from it be received in a vessel which admits of measuring its quant.i.ty, an estimate may be made of the depth due to it; and this being added to the depth given by the instrument, would probably show correctly the required depth of rain.

When snow has fallen the rain-gauge may not give a correct quant.i.ty, as a portion of it may be blown out, or a greater quant.i.ty may have fallen than the mouth will contain. In such cases, it is recommended to take a cylindrical tube and press it perpendicularly into the snow, and it will bring out with it a cylinder equal to the depth. This, when melted, will give the quant.i.ty of water which can be measured as before. The proportion of snow to water is about seventeen to one; and hail to water, about eight to one. These quant.i.ties, however, may vary according to the circ.u.mstances under which the snow or hail has fallen, and the time they have been upon the ground.

The rain-gauge should be placed as near the surface of the ground as possible; for it is a perplexing circ.u.mstance, that the rain-gauge indicates very different quant.i.ties of rain as falling upon the very same spot, according to the different heights at which it is placed. Thus it has been found, that the annual depth of rain at the top of Westminster Abbey was 12.1 inches nearly, while, on the top of a house sixteen feet lower, it was rather more than 18.1 inches, and on the ground, in the garden of the house, it was 22.6 inches. M. Arago has also found from observations made during twelve years, that on the terrace of the Observatory at Paris the annual depth was about 2 inches less than in the court thirty yards below.

It would naturally be expected from these observations, that less rain falls on high ground than at the level of the sea. Such however is not the case, except on abrupt elevations; where the elevation is made by the natural and gradual slope of the earth's surface, the quant.i.ty of rain is greater on the mountain than in the plain. Thus, on the coast of Lancashire, there is an annual fall of 39 inches; while at Easthwaite, among the mountains in the same county, the annual depth of rain amounts to 86 inches. By comparing the registers at Geneva and the convent of the Great St. Bernard, it appears that at the former place, by a mean of thirty-two years, the annual fall of rain is about 30 inches; while at the latter, by a mean of twelve years, it is a little over 60 inches.

In order to explain these remarkable differences, it must not be supposed that the clouds extend down to the ground, so as to cause more rain at the foot of Westminster Abbey than on its roof. There is no doubt that in moist weather the air contains more water near the ground than a few hundred feet above it; and probably, the same cause which determined a fall from the cloud, would also throw down the moisture floating at a low elevation. Much rain also proceeds from drifting showers, of short duration, and the current moves more slowly along the surface, and allows the drops to fall as fast as they are formed. In hilly countries, on the contrary, clouds and vapours rest on the summits without descending into the plains, and, according to some, the hills attract electricity from the clouds, and thus occasion rain to fall. Mr. Phillips supposes that each drop of rain continues to increase in size from the commencement to the end of its descent, and as it pa.s.ses successively through the moist strata of the air, obtains its increase from them; while the rain which falls on the mountain may leave these moist strata untouched, so that they may, in fact, not form rain at all.

The drops of rain are of unequal size, as may be seen from the marks made by the first drops of a shower upon any smooth surface. They vary in size from perhaps the twenty-fifth to a quarter of an inch in diameter.

It is supposed that in parting from the clouds they fall with increasing speed, until the increasing resistance of the air becomes equal to their weight, when they continue to fall with an uniform velocity. A thunder-shower pours down much faster than a drizzling rain. A flake of snow, being perhaps nine times more expanded than water, descends thrice as slow. But hailstones are often several inches in length, and fall with a velocity of seventy feet in a second, or at the rate of about fifty miles an hour, and hence the destructive power of these missiles in stripping and tearing off fruit and foliage.

The annual quant.i.ty of rain decreases from the equator to the poles, as appears from the following table, which gives the name of the station, its lat.i.tude, and the average annual number of inches of rain:-

Coast of Malabar lat. 11 30' N. 135 inches.

At Grenada, Antilles 12 126 At Cape Francois, St. 19 46' 120 Domingo At Calcutta 22 23' 81 At Rome 41 54' 39 In England 50 to 55 31 At St. Petersburgh 59 16' 16 At Uleaborg 65 30' 13

The number of rainy days, on the contrary, increases from the equator to the poles.

From 12 to 43 N. lat.-the number of rainy days in the 78 year amounts to From 43 to 46 103 From 46 to 50 134 From 50 to 60 161

The greatest depth of rain which falls in the Indian ocean is during the time when the periodical winds, called the _monsoons_, change their direction. When the winds blow directly in-sh.o.r.e the rains are very abundant, so much so that, after a continuance of twenty-four hours, the surface of the sea has been covered with a stratum of fresh water, good enough for drinking, and ships have actually filled their casks from it.

Colonel Sykes observes, that the deluge-like character of a monsoon in the Ghats of Western India, is attested by the annual amount of 302 inches, at Malcolmpait, on the Mahabuleshwar Hills.

A great depth of rain in a short time has occasionally been witnessed in Europe. At Genoa, on the 25th of October, 1822, a depth of thirty inches of rain fell in one day. At Joyeuse, on the 9th of October, 1827, thirty-one inches of rain fell in twenty-two hours. Previous to the great floods of Moray, in 1829, the rain is described as being so thick that the very air itself seemed to be descending in one ma.s.s of water upon the earth. Nothing could withstand it. The best finished windows were ineffectual against it, and every room exposed to the north-east was deluged. The smaller animals, the birds, and especially game, of all kinds, were destroyed in great numbers by the rain alone, and the mother partridge, with her brood and her mate, were found chilled to death amidst the drenching wet. It was also noticed, that, as soon as the flood touched the foundation of a dry stone wall, the sods on the top of it became as it were alive with mice, all forcing their way out to escape from the inundation which threatened their citadel; and in the stables, where the water was three feet deep, rats and moles were swimming about among the buildings.

Among the Andes it is said to rain perpetually; but in Peru it never rains, moisture being supplied during a part of the year by thick fogs, called _garuas_. In Egypt, and some parts of Arabia, it seldom rains at all, but the dews are heavy, and supply with moisture the few plants of the sandy regions.

There is a great variation in the quant.i.ty of rain that falls in the same lat.i.tude, on the different sides of the same continent, and particularly of the same island. The mean fall of rain at Edinburgh, on the eastern coast, is 26 inches; while at Glasgow, on the western coast, in nearly the same lat.i.tude, it is 40 inches. At North Shields, on the eastern coast, it is 25 inches; while at Coniston, in Lancashire, in nearly the same lat.i.tude, on the western coast, it is 85 inches.

The amount of rain in a district may be changed by destroying or forming forests, and by the inclosure and drainage of land. By thinning off the wood in the neighbourhood of Ma.r.s.eilles, there has been a striking decrease of rain in fifty years.

In Mr. Howard's observations on the climate of this country, he has found, on an average of years, that it rains every other day; that more rain falls in the night than in the day; that the greatest quant.i.ty of rain falls in autumn, and the least in winter; that the quant.i.ty which falls in autumn is nearly double that in spring; that most rain falls in October and least in February, and that May comes nearest to the mean: that one year in every five, in this country, may be expected to be extremely dry, and one in ten extremely wet.

According to Dalton, the mean annual amount of rain and dew for England and Wales is 36 inches. The mean all over the globe is stated to be 34 inches.

There seems to be some real connexion between the changes of the moon and the weather. Mr. Daniell says, "No observation is more general; and on no occasion, perhaps, is the almanac so frequently consulted as in forming conjectures upon the state of the weather. The common remark, however, goes no further than that changes from wet to dry, and from dry to wet, generally happen at the changes of the moon. When to this result of universal experience we add the philosophical reasons for the existence of tides in the aerial ocean, we cannot doubt that such a connexion exists. The subject, however, is involved in much obscurity."

At Viviers, it was observed that the number of rainy days was greatest at the first quarter, and least at the last. Mr. Howard has observed that, in this country, when the moon has south declination, there falls but a moderate quant.i.ty of rain, and that the quant.i.ty increases till she has attained the greatest northern declination. He thinks there is "evidence of a great _tidal wave_, or swell in the atmosphere, caused by the moon's attraction, preceding her in her approach to us, and following slowly as she departs from these lat.i.tudes."

Most dry climates are subject to periodical droughts. In Australia, they return after every ten or twelve years, and are then followed by excessive rains, which gradually become less and less till another drought is the consequence.

When Mr. Darwin was in South America, he pa.s.sed through a district which had long been suffering from dry weather. The first rain that had fallen during that year was on the 17th of May, when it rained lightly for about five hours. "With this shower," he says, "the farmers, who plant corn near the sea-coast, where the atmosphere is more humid, would break up the ground; with a second, put the seed in; and, if a third should fall, they would reap in the spring a good harvest. It was interesting to watch the effect of this trifling amount of moisture. Twelve hours afterwards the ground appeared as dry as ever; yet, after an interval of ten days, all the hills were faintly tinged with green patches; the gra.s.s being sparingly scattered in hair-like fibres a full inch in length.

Before this shower every part of the surface was bare as on a high road."

A fortnight after this shower had fallen, Mr. Darwin took an excursion to a part of the country to which the shower had not extended. "We had, therefore," he says, "in the first part of our journey a most faint tinge of green, which soon faded away. Even where brightest, it was scarcely sufficient to remind one of the fresh turf and budding flowers during the spring of other countries. While travelling through these deserts, one feels like a prisoner, shut up in a gloomy courtyard, longing to see something green, and to smell a moist atmosphere."

The effects of a great drought in the Pampas are thus described. "The period included between the years 1827 and 1830 is called the 'gran seco'

or the great drought. During this time so little rain fell, that the vegetation, even to the thistles, failed; the brooks were dried up, and the whole country a.s.sumed the appearance of a dusty high road. This was especially the case in the northern part of the province of Buenos Ayres, and the southern part of St. Fe. Very great numbers of birds, wild animals, cattle, and horses, perished from the want of food and water. A man told me that the deer used to come into his courtyard to the well which he had been obliged to dig to supply his own family with water; and that the partridges had hardly strength to fly away when pursued. The lowest estimation of the loss of cattle in the province of Buenos Ayres alone, was taken at one million head. A proprietor at San Pedro had previously to these years 20,000 cattle; at the end not one remained.

San Pedro is situated in the midst of the finest country, and even now again abounds with animals; yet, during the latter part of the 'gran seco' live cattle were brought in vessels for the consumption of the inhabitants. The animals roamed from their _estancias_, and wandering far to the southward, were mingled together in such mult.i.tudes that a government commission was sent from Buenos Ayres to settle the disputes of the owners. Sir Woodbine Parish informed me of another and very curious source of dispute; the ground being so long dry, such quant.i.ties of dust were blown about, that in this open country the landmarks became obliterated, and people could not tell the limits of their estates.

"I was informed by an eye-witness, that the cattle in herds of thousands rushed into the river Parana, and being exhausted by hunger they were unable to crawl up the muddy banks, and thus were drowned. The arm which runs by San Pedro was so full of putrid carca.s.ses, that the master of a vessel told me, that the smell rendered it quite impossible to pa.s.s that way. Without doubt, several hundred thousand animals thus perished in the river. Their bodies, when putrid, floated down the stream, and many in all probability were deposited in the estuary of the Plata. All the small rivers became highly saline, and this caused the death of vast numbers in particular spots, for when an animal drinks of such water it does not recover. I noticed, but probably it was the effect of a gradual increase, rather than of any one period, that the smaller streams in the Pampas were paved with bones. Subsequently to this unusual drought, a very rainy season commenced, which caused great floods. Hence it is almost certain, that some thousands of these skeletons were buried by the deposits of the very next year. What would be the opinion of a geologist viewing such an enormous collection of bones, of all kinds of animals and of all ages, thus embedded in one thick earthy ma.s.s? Would he not attribute it to a flood having crept over the surface of the land, rather than to the common order of things?"

Captain Owen mentions a curious effect of a drought on the elephants at Benguela on the western coast of Africa:-"A number of these animals had some time since entered the town in a body to possess themselves of the wells, not being able to procure any water in the country. The inhabitants mustered, when a desperate conflict ensued, which terminated in the ultimate discomfiture of the invaders, but not until they had killed one man, and wounded several others." The town is said to have a population of nearly three thousand. Dr. Malcolmson states, that during a great drought in India the wild animals entered the tents of some troops at Ellore, and that a hare drank out of a vessel held by the adjutant of the regiment.

In connexion with droughts may be mentioned a plan {133} proposed by Mr.

Espy of the United States of America, for remedying them by means of _artificial rains_. That gentleman says, that if a large body of heated air be made to ascend in a column, a large cloud will be generated, and that such cloud will contain in itself a self-sustaining power, which may move from the place over which it was formed, and cause the air over which it pa.s.ses to rise up into it and thus form more cloud and rain, until the rain may become general.

It is proposed to form this ascending column of air by kindling large fires which, Mr. Espy says, are known to produce rain. Humboldt speaks of a mysterious connexion between volcanoes and rain, and says that when a volcano bursts out in South America in a dry season, it sometimes changes it to a rainy one. The Indians of Paraguay, when their crops are threatened by drought, set fire to the vast plains with the intention of producing rain. In Louisiana, heavy rains have been known from time immemorial to succeed the conflagration of the prairies; and the inhabitants of Nova Scotia bear testimony to a similar result from the burning of their forests. Great battles are said to produce rain, and it is even stated that the spread of manufactures in a particular district deteriorates the climate of such district, the ascending current occasioned by the tall chimney of every manufactory tending to produce rain. In Manchester, for example, it is said to rain six days out of seven.

[Picture: Decorative picture of person by pool]

[Picture: Decorative picture of pastoral scene with rainbow]

CHAPTER VI.

THE RAINBOW-DECOMPOSITION OF WHITE LIGHT BY THE PRISM-FORMATION OF PRIMARY AND SECONDARY BOWS-RAINBOWS IN MOUNTAIN REGIONS-THE RAINBOW A SACRED EMBLEM-LUNAR RAINBOW-LIGHT DECOMPOSED BY CLOUDS-THEIR BEAUTIFUL COLOURS-EXAMPLES.

By means of rain and rain clouds we get that beautiful appearance so well known as the rainbow. In order to form some idea of the manner in which the rainbow is produced, it is necessary to know something of the manner in which light is composed. Sir Isaac Newton was the first philosopher who clearly explained the composition of light, as derived from the sun.

He admitted a ray of the sun into a darkened room through a small hole in the window shutters; in front of this hole he placed a gla.s.s prism, and at a considerable distance behind the prism he placed a white screen. If there had been no prism between the hole and the screen, the ray of light would have proceeded in the direction of the dotted lines, and a bright spot would have fallen upon the floor of the room, as shown in the figure. But the effect of the prism is to refract or bend the ray out of its ordinary course, and in doing so it does not produce a white spot upon the screen, but a long streak of beautiful colours, in the order marked in the figure, red being at the bottom, then orange, yellow, green, blue, indigo, and violet at the top.

[Picture: Decomposition of white light]

In order to account for the production of these colours from a ray of light, Newton supposed that such a ray is actually made up of seven distinct colours, which being mixed in proper proportions neutralize or destroy each other. In order to account for the decomposition of the ray of white light by the prism, and for the lengthened form of the _spectrum_, as it is called, he supposed that each of the seven coloured rays was capable of being bent by the prism in a different manner from the rest. Thus, in the figure, the red appears to be less bent out of the direction of the original ray than the orange-the orange less than the yellow, and so on until we arrive at the violet, which is bent most of all.

It is scarcely necessary to remark, that these views were found to be correct, except as regards the number of colours in the solar spectrum; for it is now ascertained, with tolerable certainty, that there are only three primitive or pure colours in nature, and these are _red_, _yellow_, and _blue_; and it is supposed that by mingling two or more of these colours in various proportions, all the colours in nature are produced.

Now, to apply this explanation to the production of the rainbow, which is usually seen under the following circ.u.mstances:-The observer is placed with his back to the sun, and at some distance before him rain is falling,-the air between the sun and the rain being tolerably clear. He then often sees two circular arcs or bows immediately in front of him.

The colours of the inner bow are the more striking and vivid of the two.

Each exhibits the same series of colours as in the spectrum formed by the prism; namely, _red_, _orange_, _yellow_, _green_, _blue_, _indigo_, and _violet_; but the arrangement of these colours is different in the two bows, for while in the inner bow the lower edge is violet and the upper red, in the outer bow the lower edge is red and the upper violet. The production of both bows is due to the refraction and reflexion of light, the drops of rain forming, in fact, the prism which decomposes the white light of the sun. The colours in the rainbow have the same proportional breadth as the s.p.a.ces in the prismatic spectrum. "The bow is, therefore," as Sir D. Brewster remarks, "only an infinite number of prismatic spectra, arranged in the circ.u.mference of a circle; and it would be easy, by a circular arrangement of prisms, or by covering up all the central part of a large lens, to produce a small arch of exactly the same colours. All we require, therefore, to form a rainbow, is a great number of transparent bodies capable of forming a great number of prismatic spectra from the light of the sun."

The manner in which the drops of rain act as prisms, may, perhaps, be better understood with the a.s.sistance of the following diagram. Suppose the two lower circles to represent drops of rain which a.s.sist in forming the primary bow, and the two upper circles similar drops which help to produce the secondary bow; and let S represent rays of the sun falling upon them. The rays of the sun fall upon every part of the drop; but, as those which pa.s.s through or near the centre come out on the opposite side and form a focus, they need not be taken into account. Those rays, however, which fall on the upper side of the drops, will be bent or refracted, the red rays least, and the violet most; and will fall upon the back of the drop in such a manner as to be reflected to the under part of the drop; on quitting which they will be again refracted, so as to be seen at E, where there will appear to the observer a prismatic spectrum with the red uppermost, and the violet undermost. These remarks apply to those drops only which form the upper part of the bow, but it is obvious that a similar reasoning applied to the drops to the right and left of the observer, will complete the bow. The inclination of the red ray and the violet ray to the sun's rays, is 42 2' for the red, and 40 17' for the violet, so that the breadth of the primary bow is 1 45'.