diminution, owing to the meredian altitude of the sun becoming less and less, till his rays reach the surface too obliquely to prevent nature from being clothed with perpetual ice and snow.

The varying length of time during which the sun is above and below the horizon is another cause of differential climates, as far as climate depends upon geographical position. Passing from the equator to the poles, they become increasingly marked by stronglycontrasted temperatures at different periods of the year. When the days are long, the continued solar action produces a powerful accumulation of heat, and the nights being short, comparatively little of it is lost by radiation. The effect is of course opposite in inverse circumstances. Under the equator the days and nights are always of equal length, and no great differences of temperature, or seasonal contrasts, are experienced. There is but little variation also under the tropics. But in mean and high latitudes the inequality in the duration of day and night becomes great; and as the long days coincide with the northern declination of the sun when the solar rays fall less obliquely, while the short days coincide with his southern declination when the opposite takes place, annually occurring alternations of great heat and cold are produced. The higher the latitude the more unequally is temperature diffused throughout the year. Grapes sometimes ripen in the open air at Quebec, where at an opposite season the freezing of mercury is no uncommon circumstance. The long period during which the sun remains above the horizon frequently renders the summer temperature oppressive even within the limits of the Arctic circle. In the north of Norway, lat. 70°, the thermometer has been observed to rise to above 80°. But the influence of latitude in the determination of climate is vastly modified by the configuration of the land-whether it spreads out into plains only slightly raised above the level of the sea, or is piled up into mountains towering to the clouds. It is well known that the atmosphere is not heated by transmitting the rays of the sun, but by the radiation of heat from the surface of the earth warmed by the solar beams, and chiefly by actual contact with it. Hence the temperature of the air becomes progressively lower with its height or distance from the general mass of the globe. Its density also is not uniform, but diminishes from below upwards; and rarefied air has a less capacity for heat than when it is compressed. In journeying from the equator at the sea-level, a very considerable distance must be passed before any marked diminution in the mean annual temperature will be apparent. But upon a vertical ascent of only a thousand yards, the decrement of heat is very perceptible; and the decrease proceeds with a further ascent, till at the height of about 16,000 feet in equatorial regions, a limit is reached where the thermometer never rises above the freezing-point. This is the line of constant congelation or perpetual snow, which is at its greatest height within the tropics, and from thence descends till it is met with at the lowest levels in polar latitudes. It follows that countries which are at different elevations above the sea, though in the same latitude, must have different climates; and that in the same region there may be great diversities of climate co-existing within narrow bounds, according as the surface has varying levels. Hence, while the tropical lowland is oppressively hot, and loaded with luxuriant vegetation, if moisture is not wanting, the tropical mountain rising to the height of Mont Blanc is in its higher region as cold and bare of verdure as any polar site.

Proximity to the ocean, or remoteness from it, is another influential cause in producing differential climates. The sea, not being so rapidly heated and cooled as the land, does not pass to such extremes of heat and cold, but preserves a far more uniform temperature, lower in summer and higher in winter, than what is experienced in inland districts. This character is imparted to the atmosphere in contact with the surface, and is transferred by the winds to the shores within their range. Hence maritime climates are more moderate

and equable than the continental, the cooler currents of air from the ocean tempering their summer heat, and warmer currents mitigating their winter cold. The immense evaporation also from the sea, by producing a frequently overcast sky along the shores, contributes to the same effect, the clouds moderating the solar influence in summer, and checking the radiation of heat from the land in winter. The summers of London are cooler than those of Paris, owing to the former being more exposed to maritime influences than the latter, while the winters are milder from the same cause, though the latitude is more northerly. The same is true of the seasons of Ireland as compared with those of Great Britain, that island being more fully exposed to the moderating and equalising influence of the ocean. Climates are indicated as 'excessive' which have summer and winter temperatures in violent contrast. There is a difference of about 26° between the mean temperature of the hottest and coldest months at London, of 34° at Paris, and the difference increases in proportion as the position becomes more continental. Thus it is 38° at Berlin, 43' at Warsaw, and 57° at Moscow. But the greatest known range of temperature occurs at Yakutsk, in the heart of Eastern Siberia, amounting to the fearful alternation of 106° between the months of January and July.

There are various other circumstances which exert an important influence upon climate, as the prevailing winds to which a country is exposed, the direction of mountain chains in relation to the cardinal points, the aspect or slope of the surface, the geological character of the soil, the degree of cultivation it has received, and perhaps the internal heat of the globe. These causes, along with those already mentioned, acting singly or in concert, originate the diversities of climate, defined as hot, warm, temperate, and cold, or hot and moist, cold and moist, hot and dry, cold and dry, and other varieties. If the prevalent winds that sweep over the British Isles were easterly instead of westerly, or came from the direction of the continent instead of the vast Atlantic, and from the warmer parts of it, our climate would lose its comparatively mild and uniform temperature, and that equable distribution of humidity throughout the year would be interrupted, which ordinarily contributes to render the pastures freshly green when those of the continental interior are regularly ashy or brown, parched with autumnal heat and drought. The excessive rigour of the winters in European Russia, at a latitude corresponding to that of the basin of the Thames, is in great part owing to the absence of any chain of mountains to serve as a screen from the cold winds of the Arctic Ocean. In Asiatic Russia also, where the winters are still more severe, the surface has fuller exposure to the polar blasts by sloping towards the north, while at the same time the towering protuberances of Central Asia, by having a general direction east and west, intercept the winds from the south, which would otherwise carry its warmth to mitigate the northern cold.

In defining particular zones of climate, the astronomical lines of the tropics and the polar circles are practically useless, because the average annual temperature of the atmosphere at any given point cannot be predicted alone from geographical position. Hence its distribution over the globe is shown on maps by means of isothermal lines, or lines of equal warmth, which are drawn through places with the same mean temperature, generally that of the year. Towards the equator the isotherms exhibit no great divergence from the parallels of latitude, but in extra-tropical localities their inflections and curvatures become remarkable. Thus, if we suppose a traveller to journey from London to those places in the northern hemisphere which have the same mean annual temperature, he would not travel along its parallel of latitude, 51°, but proceed on the one hand northwest through Ireland to lat. 55°, and then descend on the other side of the Atlantic to lat. 40 below New York. In the opposite direction he would pass to Vienna, lat. 48°, and to the mouth of the Danube, lat. 44°. Following the parallel of London, where the

mean temperature is 50°, that degree of heat falls gradually to 40° in the east of Europe, and to below the freezing-point in Siberia. Lines drawn on maps to connect places which have the same mean summer and winter temperatures are termed isotheral and isocheimonal, signifying equal summer, equal winter.

IV. LUMINOUS METEORS-ATMOSPHERIC ILLUSIONS.

The discharge of electricity in the atmosphere between one cloud and another, or between a series of clouds and the earth, is attended by the evolution of light and the sound of explosion, or lightning and thunder, which are precisely identical, though upon an incomparably grander scale, with the spark and snap obtained from a common electrifying machine. The cause or agent of the phenomena, called for the sake of convenience a fluid, is perfectly inscrutable in itself. It makes no appeal to the senses in a quiescent state, though apparently universally diffused, pervading the pores of all bodies, and capable of passing from one substance to another. But it may be roused from a neutral condition to become momentarily visible, display tremendous energies, and produce destructive effects, by a variety of causes, as friction, heat, and chemical action, while the reason remains in profound obscurity why these causes elicit its powers. In its geographical distribution the electric fluid seems to sympathise generally with light and heat, diminishing in energy of action from the equator to the poles. Hence thunderstorms are not only the most frequent in the tropical zone, but at the same time the most violent, the lightning flashing with a breadth and intensity, and the thunder pealing with an awfulness, of which no conception can be formed by the inhabitants of other regions. The comparative number of storms becomes less, and their character more subdued, as we recede from the tropics. In the south European countries, Italy and Greece, there are on an average about forty storms in the year, while Germany has only about twenty in the same interval, and Scandinavia not more than ten. In polar latitudes electrical explosions are still less frequent, and in some places altogether unknown. At the Faroe Islands thunder is seldom heard, and lightning is never known to do any injury. In Iceland, while lightning is not uncommon, especially in the neighbourhood of the volcanoes, thunder is rare, and was only heard once at Reikiavik in the interval between September 21, 1833, and the end of August 1835. It is stated by Giesecke, that during a residence of six years in Greenland, in latitude 70°, he only heard thunder once.

Three varieties of lightning are distinguished by Arago, which he defines lightning of the first, second, and third classes. The first of these is usually called forked-lightning, éclair en zig-zag of the French. It appears as a thin line of light, very sharply defined at the edges, and describes a zig-zag course, often bifurcates or ramifies into several divisions, though always proceeding from a single point. The irregular path may be due to the unequal conductibility of the air. The second class, known as sheet-lightning, is far less intense, and much more common. It appears as an expanded flash, occasionally opening up the whole sky to view, and showing distinctly the entire outline of the clouds. The third class, named globular-lightning, has the appearance of what are popularly termed fire-balls, globes de feu, which have a sensibly progressive motion, and remain visible for a few seconds. This kind of lightning is only seen on rare occasions after violent electrical discharges, and is perhaps to be viewed as forming no proper part of the phenomenon, but as an igneous meteor directly resulting from it in a manner which has not been explained. The lightning of the first and second classes is essentially momentary. A less duration in passing than the one-millionth part of a second is attributed to the light of electricity of high tension. In comparison with this velocity, the most rapid artificial motion that can be produced appears repose. The colour of lightning is variously reddish,

LUMINOUS METEORS-ATMOSPHERIC ILLUSIONS.

239 orange, white, and blue, verging to violet. The more electricity there is passing at the same time through the air, the whiter and more dazzling is the light. Violet and bluecoloured lightning is observed to be discharged from thunder-clouds high in the atmosphere, where the air is rarefied; and analogously the electric spark made to pass. through the receiver of an air-pump exhibits a blue or violet light in proportion as the vacuum is complete.

Lightning striking combustible substances often produces ignition. Its irresistible force is displayed when encountering on its passage to the ground objects which are bad conductors, shattering walls and roofs, vitrifying and enamelling the surfaces of rocks, and producing the fulgurites, or fulminary tubes, often observed in beds of sand and similar soils. Examples of the latter were first noticed by Pastor Herman, at Massel, in Silesia, in 1711, but their real nature was unknown. Dr Priestley described some fulgurites found while digging into the ground under a tree where a man had been killed by lightning. Dr Hentzen, in 1805, examined others in the sand-hills of Holstein, and was the first to indicate their origin. Three were inspected by Dr Buckland and Mr Greenhough, near Drigg, in Cumberland, occurring within a few yards of each other, one of which was traced downward to the depth of thirty feet. Mr Darwin observed several in a broad band of sand-hillocks near the banks of the Rio Plata. The tubes vary from half an-inch to nearly three inches in diameter, contract towards the lower extremity, and terminate in a point. They are composed of sand melted and vitrified by the action of the electric fluid, and may be artificially imitated by electrical experiments.

The loss of human life by lightning may be locally small, but it amounts in many countries to a considerable aggregate in the course of a few years. In the fourteen years, 1852-65 inclusive, 242 deaths were registered in England and Wales by lightning, of which 199 were males, and 43 were females. The numbers in each year fluctuated considerably, and indicated the periodical disturbances of the electric tension of the atmosphere. Thus, in each of the seven years, 1852-58, the deaths were 45, 10, 17, 17, 14, 18, and 26 respectively; and in the seven years following, 1859-65, they were 17, 12, 26, 3, 6, 12, and 19. The electric force which was distributed over the whole face of the country differed in tension not only at different times, but at different places. Of the 242 deaths in the fourteen years, only 3 occurred in London, whereas in the WestMidland counties, with rather less population, there were 41. In the South-Eastern counties 15 deaths occurred; in the South-Midland, 19; in the Eastern, 19; in the South-Western, 15; in the North-Midland, 31; in the North-Western, 29; in Yorkshire, 35; in the Northern counties, 19; in Monmouthshire and Wales, 16. Persons in-doors are much less likely to be struck by lightning than those who are out in the open air; and the returns show that nearly all the deaths occurred among those who were either travelling on the roads, labouring in the fields, or sheltering under trees, so that it is not surprising that the fatal flash struck men and women in the proportion of nine of the former to two of the latter.

Thunder, the report which accompanies an electric discharge, arises doubtless from the violent displacement of the air by the fluid in its passage through it, and the rush back of the air to supply the partial vacuum created. The prolonged sound, peal after peal, may be occasioned partly by reverberation, and partly by the report from different points in the track of the lightning reaching the ear in succession. Thunder is heard at varying intervals after the flash passes, owing to sound travelling at a slower rate than the luminous sensation. It makes a deep impression more by its volume than by its intensity, for the loudest thunder has never been heard at a greater distance than twelve or fourteen miles from the point of discharge. It is therefore far less intense than the report of heavy

ness.

artillery, for the cannonading at the battle of Waterloo was heard at the town of Criel, in the north of France, at the distance of about 115 miles. Lightning without thunder, generally called heat-lightning, perfectly harmless, is often observed after sunset in summer, and during the night. It is attributed by some to the air being humid, and therefore adapted for the conduction of electricity, occasioning frequent but weak discharges, so that no report is heard, and the flash is invisible by day owing to its feebleThis kind of lightning is very common in tropical countries at the commencement of the rainy season. But in some instances it appears to be due to reflection from distant thunder-storms. Kaemtz relates, that in August 1832, when heat-lightning was very frequent at Geneva, the subject was actively discussed by the Société de Physique et d'Histoire Naturelle. After the sitting, as if to test the opinions that had been advanced, the whole of the north horizon was illuminated; and some days after the newspapers were filled with accounts of the ravages of storms in Baden, Wurtemburg, and Bavaria.

In southern climes especially, during thunder-storms, lights of electric origin are occasionally seen on points and projections, and are frequently accompanied by a slight hissing noise. The luminous appearance is evidently of the same nature as the light caused by electricity streaming off from points connected with an electrical machine, forming one variety of what is called the brush discharge. It sometimes takes the shape of a bunch of feathers, or a star-like glow, or appears more simply as a pale flickering flame. It has been noticed on the top-masts and yard-arms of ships, on spears and bayonets, on bushes by the wayside, on the spires of buildings, the manes and tails of horses, the beards of men, showing the tendency of electricity to concentrate on small terminal surfaces. In showers also of rain and snow, the drops and flakes have been observed to be luminous, owing to the strong charge of electricity in the air. Professor Lampadius remarked at Freyburg an intense development of electricity during a change of weather, snow and sleet falling. There was a strong phosphorescence at the extremities of the branches of trees, the light of which was very distinct, of a bluish white colour, and the snow appeared luminous as it fell. On the east coast of the United States, in January 1817, the spectacle was very remarkable to persons out of doors, for every projecting natural object, along with the hats and gloves of passengers, were surrounded with a lively vacillating flame, which emitted a slight noise like the simmering of some liquid over a fire. In 1840, one of the French officers stationed in Algeria observed that the arms of the soldiers, when piled on the ground, exhibited nothing unusual, but when carried aloft the points of the bayonets became instantly invested with a glow. The ancients were familiar at sea with these luminosities flickering about their vessels, and augured fair weather from the appearance of twin examples of the stellar form, to which they applied the name of Castor and Pollux. In modern times they have been known to Roman Catholic mariners as St Elmo's Fire, so called after the patron saint of seamen, whose preserving presence in the tempest they are supposed to indicate. During the second voyage of Columbus, when among the West India Islands, the wind began to blow strongly in the night, and the crew considered themselves in great peril until they saw several of these lambent flames playing about the tops of the masts, and gliding along the rigging, which they hailed as a pledge of their supernatural protector being near. Fernando Columbus, the brother of the commander, records as follows: 'On the same Saturday, in the night, was seen St Elmo, with seven lighted tapers at the topmast. There was much rain and thunder. I mean to say that those lights were seen which mariners affirm to be the body of St Elmo, cuerpo sante, on beholding which they chanted many litanies and orisons, holding it for certain, that in the tempest in which he appears