coasts, or long straight thoroughfares, we must sit down. upon the ground in order to place ourselves in the layer of heated air that produces this curious effect. Then men and animals at a distance appear much taller than they should, according to the laws of perspective, and their images are seen as if reflected in a sheet of water. Several times have I witnessed this phenomenon during the hot days of summer on the sandy coast of Flanders, and also in the neighbourhood of London.1

The air of the Arctic regions, and that which lies above the snow-line on mountains, is dry and cold. When not in motion man can support its effects in spite of its exceedingly low temperature; but when in motion it cannot be resisted with impunity; and it is then absolutely necessary to seek shelter, as was vividly pointed out by the American Arctic explorer, Dr. Kane, who succumbed not long after his return from his arduous enterprise, and at a very early age. But there is a certain difference between the air of the Arctic regions and that of a corresponding latitude on the slope of a mountain, which is indicated by the structure of the plants which grow in these regions: When the air of the Arctic regions is compared with that of a

1 The results of my meteorological observations on the coast of Flanders were described to the Paris Academy of Sciences in 1857. It was there (and afterwards in Paris, and again near London) that I first witnessed the curious phenomenon of rain without clouds; and, on one or two occasions, frozen rain which fell in pear-shaped solid drops. Once also, at Ostend, I was able to convince myself that lightning occasionally occurs among clouds near the zenith, without being followed by the slightest noise of thunder.

corresponding station on the Alps, the former is found to contain much more moisture, and the leaves of the same species of plant (Saxifrage, or Salix, for instance) are invariably thicker than they are when growing in the mountainous district.

The air of the Tropics is generally hot and moist; furniture and musical instruments become unglued, and valuable violins have been known to fall to pieces in India, and the West Indies, soon after their arrival from Europe. I have already alluded to its effects upon the health.

With regard to the air in cholera epidemics, Glaisher and others have drawn attention to a peculiar blue mist which occurred in London and its neighbourhood during such an epidemic; but nothing at all satisfactory was the outcome of this observation. Early in the present century, it was noted by Figari, then Professor of Botany at Abuzabel, in Egypt,1 that during the cholera epidemic of July and August 1835, several kinds of grasses, especially Maize (Indian Corn), were killed, over large districts, by blight; and the peasants who used the leaves of these plants for their cattle, or for themselves, contracted severe illness in every case.

This gentleman published in 1867-8 an important paper on the cultivation of the opium poppy in Egypt. See Ann. pharmaceutique de Reveil et Paris, etc., Paris, 1868, p. 330; Sur la Culture du Pavot à Opium, etc., par Figari Bey.

CHAPTER XVI.

The Movements of the Atmosphere-Barometer and Thermometer -Cause of their Reverse Movements-Expansion of AirDiurnal Oscillations of the Atmosphere-Fluctuations of the Electric State of the Air-Velocities of the Wind-Cause of the Movements of the Barometer English Weather Cyclones-History of Rotatory Storms-Small Local Cyclones.

THE movements of the Atmosphere have been extensively studied in modern times. We are far away from the days of Howard and Wells, whose ingenious observations laid the basis of the modern science of Meteorology; but only men like Fitz-Roy and Maury, who have navigated the seas for many years, can hope to grapple successfully with the Physical Geography of the Atmosphere.

The prime cause of the great movements of the air is heat, that mysterious power which, confusing itself with light and electricity, may justly be termed the soul of the Universe. The terrestrial sources of heat, and that derived from solar radiation, are at present nearly equalized, but the evaporation of water (which forms about two-thirds of the Earth's surface), followed by its condensation from steam to water, and to ice, yielding up enormous amounts of latent heat, exhibits a

constant, alternant source of production, and distribution, of cold and caloric, upon which depend the various climates, the regular, periodic, and variable winds, the zones of vegetation, the currents of the ocean, and, in fact, the whole life of the globe.

The same mysterious power, called heat, or caloric, which causes the circulation of the sap in plants, and of the blood in animals, circulates also the air of the Atmosphere, and the waters of the Earth, and keeps them in constant motion; for, as I said many years ago in a former work, " movement, like matter, is universal." 1

The vane makes its rotation, in our hemisphere, in the same direction as the Sun, a fact which was perfectly well known in the most remote antiquity (see Thales, and other ancient philosophers, quoted by Dove, in his Law of Storms), and the reverse occurs in the Southern hemisphere.

The warm wind and the cold wind (originating, respectively, at the tropics and the poles) alternate one with the other. When the warm wind displaces the cold wind, the change commences in the higher regions of the air, and the barometer falls before the thermometer rises. When the cold wind takes the place of the warm wind, the change occurs in the lower strata of the air, and the thermometer falls before the barometer rises.

When cold air descends into warm air, fog is produced; the descending current beats down the smoke of our cities, and disperses it over the surface of the Earth. 1 Phipson, Familiar Letters, etc., The movements of plants.

According to the very exact researches of Magnus, of Berlin, on the expansion of air from 32° Fahr. to 212° Fahr., it follows that air at the freezing-point of water expands 4th part of its bulk for every degree of heat of Fahrenheit's scale.

Thus, 491 cubic inches of air at 32° Fahr., become 492 at 33°, 493 at 34°, and so on, increasing one cubic inch for every degree Fahrenheit.

A contraction of one cubic inch occurs for every degree below 32°. Thus, 491 cubic inches at 32° become 490 at 31°, 489 at 30°, 488 at 29°, and so on.

This shows that atmospheric air taken at the freezingpoint of water has its volume doubled at 491° Fahr.; and when heated up to 982° Fahr. (a low red heat) its volume is exactly tripled.

With regard to the diurnal oscillation of the atmosphere, the following little table is universally admitted:At 4 a.m. there occurs a first minimum of the barometer. maximum (highest).

10 a.m.

These regular oscillations of pressure are easiest perceived at the sea-coast in the equatorial regions, where they are regular enough to allow us to judge of the hour without resorting to a watch. They are difficult to put in evidence in our temperate regions, but it can be done, and they appear, like the daily magnetic variation, to be intimately connected with the position of the sun in the heavens.

The fluctuations of the electric state of the atmosphere,