The greatest height to which anyone has travelled on foot into the atmosphere, was that reached, some years ago, by the brothers (Herman, Adolph, and Robert) Schlagintweit, namely, 21,000 feet, on the 20th August 1856, on the Abi-Gumin, one of the highest summits of the Himalayas.1 Their barometer stood there at about 15 inches; they had, therefore, only half the ordinary weight of the air to support (say 7, instead of 15 lbs., to the square inch). Headache, difficulty of breathing, spitting of blood, irritation of the lungs, and great lassitude, were experienced. These symptoms gave way as soon as they reached lower ground. These travellers, like Dr. Kane in the Arctic regions, suffered less from the cold than from the wind; and they generally felt better in the morning than in the evening. Muscular action induced immense fatigue; even the act of speaking was fatiguing, and the lassitude was such that they could have fallen down and gone to sleep, for ever, upon the snow, had not supreme moral courage vanquished this great physical weakness.

Water, in the form of steam or vapour, which always exists to a certain amount in the atmosphere near the Earth's surface, is, as we said before, thrown out in the shape of cloud, or mist, as the air becomes colder. Now, as cold increases with the altitude, the vapour thus thrown out, and rendered visible where, before, it was invisible and only to be detected by our physical instruments, or by chemical analysis, assumes the form

1 Some of our British troops have since been even higher than this, in the recent border warfare in North-Western India.

of snow or ice at a certain elevation on the slopes of high mountains. At a given altitude, which varies according to a multitude of circumstances, we come upon snow, or, as it is usually termed, perpetual snow, corresponding to the perpetual ice of the Arctic regions.

The term snow-line has been given to indicate the height which forms the lower limit on a mountain slope at which this phenomenon occurs. But this is not a fixed line at any given latitude, nor at any given point of the globe; it varies, by one to three thousand feet, according to the "climate" of the locality. But, in any region. outside the Arctic zones, it is generally met with at altitudes exceeding 10,000 or 12,000 feet. On the volcano of Peuquenes (latitude 33° S.) it lies at about 15,000 feet; in the Himalayas at 12,000 to 13,000 feet; in Thibet at 16,000 feet, and so on.1

Alex. von Humboldt, who paid much attention to this subject, calls attention to the interesting fact that we are only acquainted with the lower (not the upper) limit of perpetual snow; for the mountains of the Earth do not attain to those higher regions of rarefied and dry air in which no water can possibly exist.

1 See Humboldt, Asie Centrale, vol. iii., for table of height of perpetual snow in both hemispheres, from 71° N. to 58° S. lat.

CHAPTER XVIII.

The Formation of Clouds-The Vesicular Theory-The Present Theory-Cirrus Cloud-Cumulus Cloud-Stratus CloudNimbus Cloud-Colour of Cloud and Sky as an Indication of Weather-Cause of Electric Phenomena in Thunderstorms -Formation of Snow and Hoar Frost-Formation of HailCurious Phenomenon of the "Cloud-arch"-First seen in England by the Author--Seen in Sicily by the poet GoetheAnd in the Arctic Regions by Sabine.

THE formation of clouds and their various aspects have been the subject of a considerable amount of speculation. The vapour of water is colourless, and is transparent like the air in which it rises and diffuses: it rises, because it is lighter than air (air=1000, vapour=623). But if the air is a few degrees colder than the vapour, the latter at once becomes visible, and takes a certain form.1

1 This sometimes occurs so suddenly that I remember, one evening, in Paris, the celebrated M. Babinet, member of the Institute and Bureau des Longitudes, who was a neighbour of mine, came to my rooms and brought me out on to the Place St. Sulpice to witness the variation in brightness of the star Algol, which is still a mystery to astronomers, when, just as we had succeeded in finding it, the sky became suddenly overcast with large mottled clouds, over the whole heavens. A clearing of the sky of equal rapidity is never witnessed, so that clouds can be formed by precipitation much more rapidly than

What then happens? Théodore de Saussure, and the older naturalists, asserted that the microscope solved this question; that the ingredients of clouds are minute vesicles of water filled with air, like the most minute soap bubbles it is possible to imagine. This was termed vesicular vapour; and when these little vesicles burst, they are converted into drops of rain.

Modern naturalists do not admit this "vesicular theory"; they contend that the water of the clouds is in the state of extremely minute drops which, when they coalesce, form larger drops that fall as rain. The minute drops remain suspended, just like certain powders, which are considerably heavier than water, will remain for a long time suspended in that liquid.

In the highest regions of the air these minute drops become frozen, and form tiny crystals of ice, which also remain suspended in the air for a length of time, as does volcanic dust, to which allusion has already been made.

The cirrus clouds, which appear like long wisps ("mares' tails"), at a great height in the air, are formed of these minute ice crystals. Although they usually appear motionless to persons on the flat lowlands, to those on the summit of a high mountain their movement becomes apparent, and they may be seen to travel with considerable speed. They are often six or seven

they can be dispersed by absorption or dissolution in the air. And this is just what occurs in the chemical laboratory, when a precipitate is formed instantaneously; but its re-dissolution is always much more gradual.

thousand yards high, and generally indicate a change of weather: their presence in the atmosphere, after several days of fine weather in Spring, Summer, or Autumn, announces rain within twenty-four hours; and this sign rarely fails. During the winter, if the weather be mild, they announce frost; but if cold, they indicate the approach of a thaw. The direction of their movement is often opposite to that of any clouds that may be seen beneath them.

The cumulus clouds are always lower; they appear at various heights, rising during the day as the heat of the sun increases. They make their appearance in Spring and Summer, and look like great bales of wool, or like mountains. When the sun shines on them, they resemble the snowy chains of the Alps, or the Pyrenees. When they disperse in the evening, it indicates a fine day for the morrow; but if they increase in size, or become more numerous at nightfall, and especially if cirrus cloud is seen above them, we may expect rain, or thunderstorms.

The stratus clouds form long, horizontal bands, often very wide, seen about sunset, and sometimes most gorgeously coloured. They disappear at sunrise, are common in Autumn, and rare in Spring (but they were very beautiful in London during the early part April 1895). They rise less high than the two kinds. of clouds previously mentioned.

of

Lastly, the nimbus, or rain-cloud, is usually of a uniform dark grey colour, and massive, though without any particular form.