Next day there was a violent thunder-storm. At 6 P.M. the storm commenced, and 60,000 dust-particles to the cubic inch of air were registered; but in the middle of the storm he counted only 13,000. There was a heavy fall of hail at this time, and he accounts for the diminution of dust-particles by the down-rush of purer upper air, which displaced the contaminated lower air.

At the Lake of Lucerne there was an exceptional diminution of the number in the course of an hour, viz. from 171,000 to 28,000 in a cubic inch. On looking about, he found that the direction of the wind had changed, bringing down the purer upper air to the place of observation. The bending downwards of the trees by the strong wind showed that it was coming from the upper air.

Returning to Scotland, he continued his observations at Ben Nevis and at Kingairloch, opposite Appin, Mr. Rankin using the instrument at the top of the mountain. These observations showed in general that on the mountain southerly, south-easterly, and easterly winds were more impregnated with dust-particles, sometimes containing 133,000 per cubic inch. Northerly winds brought pure air. The observations at sea-level showed a certain parallelism to those on the summit of the mountain. With a north-westerly wind the dust-particles reached the low number of 300 per cubic inch, the lowest recorded at any low-level station.

The general deductions which he made from his numerous observations during these two years are that (1) air coming from inhabited districts is always impure; (2) dust is carried by the wind to enormous distances; (3) dust rises to the tops of mountains during the day; (4) with much dust there is much haze; (5) high humidity causes great thickness of the atmosphere, if accompanied by a great amount of dust, whereas there is no evidence that humidity alone has any effect in producing thickness; (6) and there is generally a high amount of dust with high temperature, and a low amount of dust with low temperature.

CHAPTER VIII

A FOG-COUNTER

Next to the enumeration of the dust-particles in the atmosphere is the marvellous accuracy of counting the number of particles in a fog. The same ingenious inventor has constructed a fog-counter for the purpose; and the number of fog-particles in a cubic inch can be ascertained. This instrument consists of a gla.s.s micrometer divided into squares of a known size, and a strong microscope for observing the drops on the stage. The s.p.a.ce between the micrometer and the microscope is open, so that the air pa.s.ses freely over the stage; and the drops that fall on its surface are easily seen. These drops are very small; many of them when spread on the gla.s.s are no more than the five-hundredth of an inch in diameter.

In observing these drops, the attention requires to be confined to a limited area of the stage, as many of the drops rapidly evaporate, some almost as soon as they touch the gla.s.s, whilst the large ones remain a few seconds.

In one set of Dr. Aitken's observations, in February 1891, the fog was so thick that objects beyond a hundred yards were quite invisible. The number of drops falling per second varied greatly from time to time. The greatest number was 323 drops per square inch in one second. The high number never lasted for long, and in the intervals the number fell as low as 32, or to one-tenth.

If we knew the size of these drops, we might be able to calculate the velocity of their fall, and from that obtain the number in a cubic inch.

An ingenious addition is put to the instrument in order to ascertain this directly. It is constructed so as to ascertain the number of particles that fall from a known height. Under a low-power microscope, and concentric with it, is mounted a tube 2 inches long and 1-1/2 inch in diameter, with a bottom and a cover, which are fixed to an axis parallel with the axis of the tube, so that, by turning a handle, these can be slid sideways, closing or opening the tube at both ends when required. In the top is a small opening, corresponding to the lens of the microscope, and in the centre of the bottom is placed the observing-stage illumined by a spot-mirror. The handle is turned, and the ends are open to admit the foggy air. The handle is quickly reversed, and the ends are closed, enabling the observer to count on the stage all the fog-particles in the two inches of air over it.

The number of dust-particles in the air which become centres of condensation depends on the rate at which the condensation is taking place. The most recent observations show that quick condensation causes a large number of particles to become active, whereas slow condensation causes a small number. After the condensation has ceased, a process of differentiation takes place, the larger particles robbing the smaller ones of their moisture, owing to the vapour-pressure at the surface of the drops of large curvature being less than at the surface of drops of smaller curvature.

By this process the particles in a cloud are reduced in number; the remaining ones, becoming larger, fall quicker. The cloud thus becomes thinner for a time. A strong wind, suddenly arising, will cause the cloud-particles to be rapidly formed: these will be very numerous, but very small--so small that they are just visible with great care under a strong magnifying lens used in the instrument. But in slowly formed clouds the particles are larger, and therefore more easily visible to the naked eye.

Though the particles in a fog are slightly finer, the number is about the same as in a cloud--that is, generally. As clouds vary in density, the number of particles varies. Sometimes in a cloud one cannot see farther than 30 yards; whereas in a few minutes it clears up a little, so that we can see 100 yards. Of course, the denser the cloud the greater the number of water-particles falling on the calculating-stage of the instrument.

Very heavy falls of cloud-particles seldom last more than a few seconds, the average being about 325 on the square inch per second, the maximum reaching to 1290. This is about four times the number counted in a fog.

Yet the particles are so very small that they evaporate instantly when they reach a slight increase of temperature.

CHAPTER IX

FORMATION OF CLOUDS

In our ordinary atmosphere there can be no clouds without dust. A dust-particle is the nucleus that at a certain humidity becomes the centre of condensation of the water-vapour so as to form a cloud-particle; and a collection of these forms a cloud.

This condensation of vapour round a number of dust-particles in visible form gives rise to a vast variety of cloud-shapes. There are two distinct ways in which the formation of clouds generally takes place. Either a layer of air is cooled in a body below the dew-point; or a ma.s.s of warm and moist air rises into a ma.s.s which is cold and dry. The first forms a cloud, called, from being a layer, _stratus_; the second forms a cloud, called, from its heap appearance, _c.u.mulus_. The first is widely extended and horizontal, averaging 1800 feet in height; the second is convex or conical, like the head of a sheaf, increasing upward from a level base, averaging from 4500 feet to 6000 feet in height.

There are endless combinations of these two; but at the height of 27,000 feet, where the cloud-particles are frozen, the structure of the cloud is finer, like "mares' tails," receiving the name _cirrus_. When the cirrus and c.u.mulus are combined, in well-defined roundish ma.s.ses, what is familiarly described as a "mackerel sky" is beautifully presented. The dark ma.s.s of cloud, called _nimbus_, is the threatening rain-cloud, about 4500 feet in height.

At the International Meteorological Conference at Munich, in 1892, twelve varieties of clouds were cla.s.sified, but those named above are the princ.i.p.al. In a beautiful sunset one can sometimes notice two or three distances of clouds, the sun shedding its gold light on the full front of one set, and only fringing with vivid light the nearer range.

Although no man has wrought so hard as Dr. Aitken to establish the principle that clouds are mainly due to the existence of dust-particles which attract moisture on certain conditions, yet even twenty years ago he said that it was probable that sunshine might cause the formation of nuclei and allow cloudy condensation to take place where there was no dust.

Under certain conditions the sun gives rise to a great increase in the number of nuclei. Accordingly, he has carefully tested a few of the ordinary const.i.tuents and impurities in our atmosphere to see if sunshine acted on them in such a way as to make them probable formers of cloud-particles.

He tested various gases, with more or less success. He found that ordinary air, after being deprived of its dust-particles and exposed to sunshine, does not show any cloudy condensation on expansion; but, when certain gases are in the dustless air, a very different result is obtained.

He first used ammonia, putting one drop into six cubic inches of water in a flask, and sunning this for one minute; the result was a considerable quant.i.ty of condensation, even with such a weak solution. When the flask was exposed for five minutes, the condensation by the action of the sunshine was made more dense.

Hydrogen peroxide was tested in the same way, and it was found to be a powerful generator of nuclei. Curious is it that sulphurous acid is puzzling to the experimentalist for cloud formation. It gives rise to condensation in the dark; but sunshine very conclusively increases the condensation.

Chlorine causes condensation to take place without supersaturation; sulphuretted hydrogen (which one always a.s.sociates with the smell of rotten eggs) gives dense condensation after being exposed to sunshine.

Though the most of these nuclei, due to the action of sunshine in the gases, remain active for cloudy condensation for a comparatively short s.p.a.ce of time--fifteen minutes to half-an-hour--yet the experiments show that it is possible for the cloudy condensation to take place in certain circ.u.mstances in the absence of dust. This seems paradoxical in the light of the former beautiful experiments; but, in ordinary circ.u.mstances, dust is needed for the formation of clouds. However, supposing there is any part of the upper air free from dust, it is now found possible, when any of these gases experimented on be present, for the sun to convert them into nuclei of condensation, and permit of clouds being formed in dustless air, miles above the surface of the earth.

In the lower atmosphere there are always plenty of dust-particles to form cloudy condensation, whether the sun shines or not. These are produced by the waste from the millions of meteors that daily fall into the air.

But in the higher atmosphere, clouds can be formed by the action of the sun's rays on certain gases. This is a great boon to us on the earth; for it a.s.sures us of clouds being ever existing to defend us from the sun's extra-powerful rays, even when our atmosphere is fairly clear. This is surely of some meteorological importance.

CHAPTER X

DECAY OF CLOUDS

From the earliest ages clouds have attracted the attention of observers.

Varied are their forms and colours, yet in our atmosphere there is one law in their formation. Cloud-particles are formed by the condensation of water-vapour on the dust-particles invisibly floating in the atmosphere, up to thousands--and even millions--in the cubic inch of air.

But observers have not directed their attention so much to the decay of clouds--in fact, the subject is quite new. And yet how suggestive is the subject!

The process of decay in clouds takes place in various ways. A careful observer may witness the gradual wasting away and dilution into thin air of even great stretches of cloud, when circ.u.mstances are favourable. In May 1896 my attention was particularly drawn to this at my manse in Strathmore. In the middle of that exceptionally sultry month, I was arrested by a remarkable transformation scene. It was the hottest May for seventy-two years, and the driest for twenty-five years. The whole parched earth was thirsting for rain. All the morning my eyes were turned to the clouds in the hope that the much-desired shower should fall. Till ten o'clock the sun was not seen, and there was no blue in the sky. Nor was there any haze or fog.

But suddenly the sun shone through a thinner portion of the enveloping clouds, and, to the north, the sky began to open. As if by some magic spell there was, in a quarter of an hour, more blue to be seen than clouds. At the same time, near the horizon, a haze was forming, gradually becoming denser as time wore on. In an hour the whole clouds were gone, and the glorious...o...b..of day dispelled the moisture to its thin-air form.

This was a pointed and rapid ill.u.s.tration of the decay from cloud-form to haze, and then to the pure vapoury sky. It was an instance of the _reverse_ process. As the sun cleared through, the temperature in the cloud-land rose and evaporation took place on the surface of the cloud-particles, until by an untraceable, but still a gradual process through fog, the haze was formed. Even then the heat was too great for a definite haze, and the water-vapour returned to the air, leaving the dust-particles in invisible suspension.

But clouds decay in another way. This I will ill.u.s.trate in the next chapter on "It always rains."

What strikes a close observer is the difference of structure in clouds which are in the process of formation and those which are in the process of decay. In the former the water-particles are much smaller and far more numerous than in the latter. While the particles in clouds in decay are large enough to be seen with the unaided eye, when they fall on a properly lighted measuring table, they are so small in clouds in rapid formation that the particles cannot be seen without the aid of a strong magnifying gla.s.s.

Observers have a.s.sumed that the whole explanation of the fantastic shapes taken by clouds is founded on the process of formation; but Dr. Aitken has pointed out that ripple-marked clouds, for instance, have been clouds of decay. When what is called a cirro-stratus cloud--mackerel-like against the blue sky--is carefully observed in fine weather, it will be found that it frequently changes the ripple-marked cirrus in the process of decay to vanishing. Where the cloud is thin enough to be broken through by the clear air that is drawn in between the eddies, the ripple markings get nearer and nearer the centre, as the cloud decays. And, at last, when nearly dissolved, these markings are extended quite across the cloud.

Whether, then, we consider the cases of clouds gradually melting away back into their original state of blue water-vapour, or the constant fine raining from clouds and re-formation by evaporation, or the transformation of such clouds as the cirro-stratus into the ripple-marked cirrus, we are forced to the conclusion that in clouds there is not always development, but sometimes degeneration; not always formation, but sometimes decay.