Meteorology.

by J. G. M'Pherson.

CHAPTER I

INTRODUCTION

Though by familiarity made commonplace, the "weather" is one of the most important topics of conversation, and has constant bearings upon the work and prospects of business-men and men of pleasure. The state of the weather is the pa.s.sword when people meet on the country road: we could not do without the humble talisman. "A fine day" comes spontaneously to the lips, whatever be the state of the atmosphere, unless it is peculiarly and strikingly repulsive; then "A bitter day" would take the place of the expression. Yet I have heard "_Terrible_ guid wither" as often as "_Terrible_ bad day" among country people.

Scarcely a friendly letter is penned without a reference to the weather, as to what has been, is, or may be. It is a new stimulus to a lagging conversation at any dinner-table. All are so dependent on the weather, especially those getting up in years or of delicate health.

I remember, when at Strathpeffer, the great health-resort in the North of Scotland, in 1885, an anxious invalid at "The Pump" asking a weather-beaten, rheumatic old gamekeeper what sort of a day it was to be, considering that it had been wet for some time. The keeper crippled to the barometer outside the doorway, and returned with the matter-of-fact answer: "She's faurer doon ta tay nur she wa.s.s up yestreen." The barometer had evidently fallen during the night. "And what are we to expect?" sadly inquired the invalid. "It'll pe aither ferry wat, or mohr rain"--a poor consolation!

Most men who are bent on business or pleasure, and all dwellers in the country who have the instruments, make a first call at the barometer in the lobby, or the aneroid in the breakfast-parlour, to "see what she says." A good rise of the black needle (that is, to the right) above the yellow needle is a source of rejoicing, as it will likely be clear, dry, and hard weather. A slight fall (that is, to the left) causes anxiety as to coming rain, and a big depression forebodes much rain or a violent storm of wind. In either case of "fall," the shutters come over the eyes of the observer. Next, even before breakfast, a move is made to the self-registering thermometer (set the night before) on a stone, a couple of feet above the gra.s.s. A good reading, above the freezing-point in winter and much above it in summer, indicates the absence of killing rimes, that are generally followed by rain. A very low register accounts for the feeling of cold during the night, though the fires were not out; and predicts precarious weather. Ordinarily careful observers--as I, who have been in one place for more than thirty years--can, with the morning indications of these two instruments, come pretty sure of their prognostics of the day's weather. Of course, the morning newspaper is carefully scanned as to the weather-forecasts from the London Meteorological Office--direction of wind; warm, mild, or cold; rain or fair, and so on--and in general these indications are wonderfully accurate for twenty-four hours; though the "three days'" prognostics seem to stretch a point. We are hardly up to that yet.

The lower animals are very sensitive as to the state of approaching extremes of weather. "Thae sea bea.s.s," referring to sea-gulls over the inland leas during ploughing, are ordinary indicators of stormy weather.

Wind is sure to follow violent wheelings of crows. "Beware of rain" when the sheep are restive, rubbing themselves on tree stumps. But all are familiar with Jenner's prognostics of rain.

Science has come to the aid of ordinary weather-lore during the last twenty years, by leaps and bounds. Time-honoured notions and revered fictions, around which the hallowed a.s.sociations of our early training fondly and firmly cling, must now yield to the exact handling of modern science; and with reluctance we have to part with them. Yet there is in all a fascination to account for certain ordinary phenomena. "The man in the street," as well as the strong reading man, wishes to know the "why"

and the "how" of weather-forecasting. They are anxious to have weather-phenomena explained in a plain, interesting, but accurate way.

The freshness of the marvellous results has an irresistible charm for the open mind, keen for useful information. The discoveries often seem so simple that one wonders why they were not made before.

Until about twenty years ago, Meteorology was comparatively far back as a science; and in one important branch of it, no one has done more to put weather-lore on a scientific basis than Dr. John Aitken, F.R.S., who has very kindly given me his full permission to popularise what I like of his numerous and very valuable scientific papers in the _Transactions of the Royal Society of Edinburgh_. This I have done my best to carry out in the following pages. "The way of putting it" is my only claim.

Many scientific men are decoyed on in the search for truth with a spell unknown to others: the antic.i.p.ation of the results sometimes amounts to a pa.s.sion. Many wrong tracks do they take, yet they start afresh, just as the detective has to take several courses before he hits upon the correct scent. When they succeed, they experience a pleasure which is indescribable; to them fame is more than a mere "fancied life in others'

breath."

Dr. Aitken's continued experiments, often of rare ingenuity and brilliancy, show that no truth is altogether barren; and even that which looks at first sight the very simplest and most trivial may turn out fruitful in precious results. Small things must not be overlooked, for great discoveries are sometimes at a man's very door. Dr. Aitken has shown us this in many of his discoveries which have revolutionised a branch of meteorology. Prudence, patience, observing power, and perseverance in scientific research will do much to bring about unexpected results, and not more so in any science than in accounting for weather-lore on a rational basis, which it is in the power of all my readers to further.

"The old order changeth, giving place to new." With kaleidoscopic variety Nature's face changes to the touch of the anxious and reverent observer.

And some of these curious weather-views will be disclosed in these pages, so as, in a brief but readable way, to explain the weather, and lay a safe basis for probable forecastings, which will be of great benefit to the man of business as well as the man of pleasure.

"Felix, qui potuit rerum cognoscere causas."

--VIRGIL.

CHAPTER II

THE FORMATION OF DEW

The writer of the Book of Job gravely asked the important question, "Who hath begotten the drops of dew?" We repeat the question in another form, "Whence comes the real dew? Does it fall from the heavens above, or does it rise from the earth beneath?"

Until about the beginning of the seventeenth century, scientific men held the opinion of ordinary observers that dew fell from the atmosphere. But there was then a reaction from this theory, for Nardius defined it as an exhalation from the earth. Of course, it was well known that dew was formed by the precipitation of the vapour of the air upon a colder body.

You can see that any day for yourself by bringing a gla.s.s of very cold water into a warm room; the outer surface of the gla.s.s is dimmed at once by the moisture from the air. M. Picket was puzzled when he saw that a thermometer, suspended five feet above the ground, marked a lower temperature on clear nights than one suspended at the height of seventy-five feet; because it was always supposed that the cold of evening descended from above. Again he was puzzled when he observed that a buried thermometer read higher than one on the surface of the ground. Until recently the greatest authority on dew was Dr. Wells, who carefully converged all the rays of scientific light upon the subject. He came to the conclusion that dew was condensed out of the air.

But the discovery of the true theory was left to Dr. John Aitken, F.R.S., a distinguished observer and a practical physicist, of whom Scotland has reason to be proud. About twenty years ago he made the discovery, and it is now accepted by all scientific men on the Continent as well as in Great Britain. What first caused him to doubt Dr. Wells' theory, so universally accepted, that dew is formed of vapour existing at the time in the air, and to suppose that dew is mostly formed of vapour rising from the ground, was the result of some observations made in summer on the temperature of the soil at a small depth under the surface, and of the air over it, after sunset and at night. He was struck with the unvarying fact that the ground, a little below the surface, was warmer than the air over it. By placing a thermometer among stems below the surface, he found that it registered 18 Fahr. higher than one on the surface. So long, then, as the surface of the ground is above the dew-point (_i.e._ the temperature when dew begins to be formed), vapour must rise from the ground; this moist air will mingle with the air which it enters, and its moisture will be condensed and form dew, whenever it comes in contact with a surface cooled below the dew-point.

You can verify this by simple experiments. Take a thin, shallow, metal tray, painted black, and place it over the ground after sunset. On dewy nights the _inside_ of the tray is dewed, and the gra.s.s inside is wetter than that outside. On some nights there is no dew outside the tray, and on all nights the deposit on the inner is heavier than that on the outside.

If wool is used in the experiments, we are reminded of one of the forms of the dewing of Gideon's fleece--the fleece was bedewed when all outside was dry.

You therefore naturally and rightly come to the conclusion that far more vapour rises out of the ground during the night than condenses as dew on the gra.s.s, and that this vapour from the ground is trapped by the tray.

Much of the rising vapour is generally carried away by the pa.s.sing wind, however gentle; hence we have it condensed as dew on the roofs of houses, and other places, where you would think that it had fallen from above. The vapour rising under the tray is not diluted by the mixture with the drier air which is occasioned by the pa.s.sing wind; therefore, though only cooled to the same extent as the air outside, it yields a heavier deposit of dew.

If you place the tray on bare ground, you will find on a dewy night that the inside of the tray is quite wet. On a dewy night you will observe that the under part of the gravel of the road is dripping wet when the top is dry. You will find, too, that around pieces of iron and old implements in the field, there is a very marked increase of gra.s.s, owing to the deposit of moisture on these articles--moisture which has been condensed by the cold metal from the vapour-charged air, which has risen from the ground on dewy nights.

But all doubt upon this important matter is removed by a most successful experiment with a fine balance, which weighs to a quarter of a grain. If vapour rises from the ground for any length of time during dewy nights, the soil which gives off the vapour must lose weight. To test this, cut from the lawn a piece of turf six inches square and a quarter of an inch thick. Place this in a shallow pan, and carefully note the weight of both turf and pan with the sensitive balance. To prevent loss by evaporation, the weighing should be done in an open shed. Then place the pan and turf at sunset in the open cut. Five hours afterwards remove and weigh them, and it will be found that the turf has lost a part of its weight. The vapour which rose from the ground during the formation of the dew accounts for the difference of weight. This weighing-test will also succeed on bare ground.

When dealing with h.o.a.r-frost, which is just frozen dew, we shall find visible evidence of the rising of dew from the ground.

You know the beautiful song, "Annie Laurie," which begins with--

"Maxwelton's braes are bonnie, Where early fa's the dew"--

well, you can no longer say that the dew "falls," for it rises from the ground. The song, however, will be sung as sweetly as ever; for the spirit of true poetry defies the cold letter of science.

CHAPTER III

TRUE AND FALSE DEW

Ever since men could observe and think, they have admired the diamond globules sparkling in the rising sun. These "dew-drops" were considered to be shed from the bosom of the morn into the blooming flowers and rich gra.s.s-leaves. Ballantine's beautiful song of Providential care tells us that "Ilka blade o' gra.s.s keps it's ain drap o' dew."

But, alas! we have to bid "good-bye" to the appellation "dew-drop." What was popularly and poetically called dew _is not dew at all_. Then what is it?

On what we have been accustomed to call a "dewy" night, after the brilliant summer sun has set, and the stars begin to peep out of the almost cloudless sky, let us take a look at the produce of our vegetable garden. On the broccoli are found glistening drops; but on the peas, growing next them, we find nothing.

A closer examination shows us that the moisture on the plants is not arranged as would be expected from the ordinary laws of radiation and condensation. There is no generally filmy appearance over the leaves; the moisture is collected in little drops placed at short distances apart, along the edges of the leaves all round.

Now place a lighted lantern below one of the blades of the broccoli, and a revelation will be made. The brilliant diamond-drops that fringe the edge of the blade are all placed at the points where the nearly colourless veins of the blade come to the outer edge. The drops are not dew at all, but the exudation of the healthy plant, which has been conveyed up these veins by strong root-pressure.

The fact is that the root acts as a kind of force-pump, and keeps up a constant pressure inside the tissues of the plant. One of the simplest experiments suggested by Dr. Aitken is to lift a single gra.s.s-plant, with a clod of moist earth attached to it, and place it on a plate with an inverted tumbler over it. In about an hour, drops will begin to exude, and the tip of nearly every blade will be found to be studded with a diamond-like drop.

Next subst.i.tute water-pressure. Remove a blade of broccoli and connect it by means of an india-rubber tube with a head of water of about forty inches. Place a gla.s.s receiver over it, so as to check evaporation, and leave it for an hour. The plant will be found to have excreted water freely, some parts of the leaves being quite wet, while drops are collected at the places where they appeared at night.

If the water pressed into the leaf is coloured with aniline blue, the drops when they first appear are colourless; but before they grow to any size, the blue appears, showing that little water was held in the veins.

The whole leaf soon gets coloured of a fine deep blue-green, like that seen when vegetation is rank; this shows that the injected liquid has penetrated through the whole leaf.

Again, the surfaces of the leaves of these drop-exuding plants never seem to be wetted by the water. It is because of the rejection of water by the leaf-surface that the exuded moisture from the veins remains as a drop.