contact with the warm earth that derive most heat from the sun's rays. In addition to all this, the radiation, or loss of heat, goes on more rapidly, there being no solid objects around to return it, and the rarer air opposing less obstacle to its escape. At considerable elevations, too, there is proportionally less vapour in the air; and as it is the chief obstructor of radiation, objects in high regions are exposed naked, as it were, to the cold of the outer universe. At a certain height over every place, water will freeze, and if a mountain rise to this height, it will be covered with snow. The height over any place where water must be frozen at all seasons is called the snow-line, the diminishes as the latitude increases. At a certain high polar latitude, it reaches the mean sea-level; that is to say, the ground at that level is eternally clad with snow.* The insight into the causes of atmospheric changes afforded by these charts, may be judged of from what Mr Buchan says in describing that for January: 'Over the North Atlantic occurs an extensive diminution of pressure, which deepens north-altitude of which is greatest at the equator, and wards till the greatest depression, 29'5, is reached in Iceland, or perhaps in a slightly lower depression nearly midway between that island and Spitzbergen. The widening of the isobarometric curves of 296 and 29.7 inches to the westward over Greenland, and to the eastward over the north of Norway and Russia, is an interesting feature of this area of low pressure. The low pressure of this region is due to the saturated state of its atmosphere and to the copious rainfall resulting from it. The flow of the Gulf-stream north-eastwards through the Atlantic to at least beyond Spitzbergen, and the larger amount of vapour poured into the atmosphere from its warmer waters, tends still further to lower the pressure. It is this low pressure over the North Atlantic, together with the high pressure to the eastward over Asia, which forms the key to the explanation of the winter climate of Europe.' Distribution of Terrestrial Temperature.-The temperature of the earth differs not only in different regions of its surface, but at different elevations above or below the surface. In ascending a mountain or into the air in a balloon, the thermometer, as a rule, falls. The rate generally allowed is 1° of Fahr. for every 300 feet. The increase of cold at high elevations arises from several causes. One cause is the rarefaction of the atmosphere, which takes place on ascending (see No. 15). Under ordinary circumstances, when a gas is allowed to expand, its temperature falls. This used to be explained by saying, that a gas when rarefied has a greater capacity for heat, and therefore requires a greater absolute quantity to keep it at a certain temperature. There is really, however, no change of specific heat, for it is possible, under certain conditions, to rarefy a gas without any loss of temperature. The real cause is that the gas in expanding performs work; it puts itself in motion to occupy the wider space, and no motion can be caused without expending some form of energy. The temperature of the gas depends upon some kind of oscillating motion among its molecules; part of this molecular motion is converted into a motion of translation, and the energy or heat of the gas is diminished by the amount thus expended. Another cause is found in the fact, that the sun's rays have little effect on the atmosphere, especially when dry, and only give out their full heat when they strike solid objects. It is thus the lower strata of the air that are in According to the experiments made by Mr Glaisher in balloons, the diminution of temperature is 72 F. for the first thousand feet, but only 5.3° F. between the first and second thousand. From 14,000 to 15,000, it is reduced to 2.1°. The law, however, of loss of temperature by elevation is subject to important exceptions. In winter, the earth is losing more heat by radiation than it derives from the sun's rays; therefore, in a dry, calm, clear night, the surface rapidly loses heat, and the stratum of air in contact with it is thus chilled below the general temperature of the atmosphere. The effect is most marked at short heights above the surface. The air in contact with the ground is frequently 15° or 20° below the air four feet above the ground; above four feet the differences are comparatively small. It is evident from all this, that in comparing thermometers, it is important that they be placed at the same height above the surface. The effect of this chilling of the lower stratum of the atmosphere is modified by the configuration of the surface. From off heights and slopes, the chilled air flows down into the low-lying grounds, and there accumulates. 'This explains,' says Mr Buchan, 'why vapour becomes visible so frequently in low places, whilst adjoining eminences are clear; and the same fact instinct has made known to cattle and sheep, which generally prefer to rest during night on knolls and other eminences. Along most of the water-courses of Great Britain, during the memorable frost of Christmas 1860, laurels, araucarias, and other trees growing below a certain height were destroyed, but above that height they escaped.' The variations of temperature below the surface belong rather to Geology and Physical Geo-. graphy. We have now to consider how heat is distributed over the earth's surface horizontally. The causes of the varying quantities of heat enjoyed by dif ferent places have already been described in a general way. The results of these causes, as modified by the varying relations of land and water, are made visible to the eye by means of charts having lines drawn through all places having the same temperature. These lines are The snow-line is found at various heights, according to latitude, proximity to the sea, and other causes, which affect the found at an elevation of about 17,000 feet; in the Swiss Alps, at general climate of the region. In the Himalaya and Andes, it is 8500 feet; and in the Scandinavian range, at 3500 feet. Generally, in those countries which are near the equator, the snow-line is found about 16,000 feet, or three miles above the sea-level : about the 45th parallel in either hemisphere, it occurs at an elevation of gooo feet; under 60 of latitude, at 5000 feet or thereby : under 70° latitude, at 1000 feet; and under 80°, the snow-line comes down to the mean sea-level; for countries which are 10° distant from the poles are covered with snow all the year round. called isothermals (Gr. isos, equal, therme, heat), I when they mark mean annual temperature; isotherals (Gr. theros, summer), when they mark | the hottest month, July; and isocheimals, or isoeither the mean of the summer months or that of chimenals (Gr. cheima or chima, winter), in regard to winter. Fig. I is such a chart, exhibiting the | while fig. 2 exhibits the hottest and coldest distribution of the mean annual temperature; months; the dotted lines marking the mean temperature of January, and the solid lines that Of ocean-currents affecting temperature, the most marked and important is the Gulf-stream in the North Atlantic, which, by conveying warm water to the arctic regions, pushes the isothermals many degrees to the northward. There is a similar, though much feebler, current passing from the North Pacific to the Arctic Sea through Behring's Strait, and there, accordingly, the isothermals are pushed a little to the northward. An opposite effect is produced by two cold currents from the Antarctic Ocean, flowing, the one along the coast of Peru, the other along the west coast of Africa. greatest cold lies near the eastern part of the continent of Asia. On the other hand, in July, the interior of continents is much warmer than their western parts. Hence the interior and eastern parts of Asia and America are characterised by extreme climates, and the western parts by equable climates. Thus, at Yakutsk, in Siberia, the July temperature is 62°2, and the January -43°8, the difference being 1060; whilst at Dublin these are respectively 60°.8 and 38°5, the difference being only 22°3. This constitutes the most important distinction of climates, both as respects vegetable and animal life. On man especially the effect is very great-the severity of the strain of extreme climates on his system being shewn by the rapidly increasing death-rate as the difference between the July and January temperatures increases. Winds and Storms.-Winds are classed as Constant, Periodical, and Variable. The Tradewinds and Return Trades constitute the first class; Sea and Land breezes, and Monsoons form the periodical class, and both have been considered under the general movements of the atmosphere. While the decrease of temperature in advancing towards the poles corresponds in a general way to what may be called the solar climate, there are deviations brought about by disturbing causes too important to be overlooked. The chief of these Variable winds depend on purely local or temdisturbing causes are (1) the currents of the sea;porary causes, such as the nature of the ground, and (2) the prevailing winds. covered with vegetation or bare; the physical configuration of the surface, level or mountainous; the vicinity of the sea or lakes; and the passage of storms. The hot, suffocating wind peculiar to Africa and Western Asia is known by the name of the Simoom; on the coast of Guinea it is called the Harmattan. A similar wind in Sicily and Italy is called the Sirocco, and in Spain the Solano. The East Winds which prevail in the British Islands in spring are part of the great polar current which at that season descends over Europe through Russia. Their origin explains their dryness and unhealthiness. It is a prevalent notion that the east winds in this country are damp. It is quite true that many easterly winds are peculiarly damp; all that prevail in the front part of storms are very damp and rainy, and soon shift round to some westerly point. But the genuine east wind, which is the dread of the nervous and of invalids, does not shift to the west, and is specially and intolerably dry. Deaths from brain-diseases and consumption reach the maximum in Great Britain during the prevalence of east winds. The Etesian Winds are northerly winds which prevail in summer over the Mediterranean Sea. They are caused by the great heat of North Africa at this season, and consist in a general flow of the air of the cooler Mediterranean to the south, to take the place of the heated air which rises from the sandy deserts. The Mistral is a steady, violent north-west wind, felt particularly at Marseille and the south-east of France, blowing down on the Gulf of Lyons. Since winds bring with them the temperature of the regions they have crossed, the equatorial current is a warm wind, and the polar a cold wind; also winds arriving from the ocean are not subject to such variation of temperature during the year as winds from a continent. As an atmosphere loaded with vapour obstructs both solar and nocturnal radiation, it follows that moist winds are accompanied with a warm temperature in winter, and a cool temperature in summer; and dry winds with cold winters and hot summers. The direction of mountain-ranges is also an important element to be taken into account in estimating the influence of winds on temperature. These considerations explain the position of the isothermals in the north temperate zone, where the prevailing wind is the south-west or anti-trade. In January, the western parts of each continent enjoy a comparatively high temperature, from their proximity to the ocean, whose high temperature the winds waft thither; and they are further protected from extreme cold by their moist atmosphere and clouded skies. But in the interior of the continents it is otherwise; for the winds getting colder as they advance, and being deprived of their moisture as they cross the mountains in the west, the soil is exposed to the full effects of radiation during the long winter nights, and as a consequence, the temperature rapidly falls. In the centre of Siberia, the January temperature falls to -40°, which is 9° colder than the coldest part of the American continent; and this centre of Lord Bacon remarked that the wind most frequently veers with the sun's motion, or passes round the compass in the direction of N., N.E., E., S.E., S., S.W., W., and N.W., to N. This follows in consequence of the influence of the earth's rotation in changing the direction of the wind. Professor Dové of Berlin has the merit of having first propounded the Law of the Rotation of the Winds, and proved that the whole system of atmospheric currents-the constant, the periodical, and the variable winds-obey the influence of the earth's rotation. A с The force of the wind is measured by Anemometers, of which some measure the velocity, by the revolution of vanes, and others the pressure. Of the latter kind is Lind's Windgauge, represented in the figure. When the instrument is used, water is poured into the tubes until the level in both stands at the middle of the scale. When no disturbing force acts upon either column of liquid, the level of both is accurately the same; but when the mouth of the tube AB is turned towards the wind, the column in AB is pressed downwards, and that in CD rises proportionably, and the difference of the heights of the two columns gives the column of water which the force of the wind sustains, and from this the pressure on a square foot is readily calculated. | B E D Dr Lind's Anemometer. двам The following are a few velocities of wind, translated into popular language: 7 miles an hour is a gentle air; 14 miles, a light breeze; 21 miles, a good steady breeze; 40 miles, a gale; 60 miles, a heavy storm; and 80 to 100 miles, a hurricane sweeping everything before it. We also add a few comparisons of velocity and pressure: 5 miles an hour is a pressure of 2 oz. on the square foot; 10 miles, 1b.; 20 miles, 2 lbs. ; 30 miles, 4 lbs.; 40 miles, 8 lbs. ; 51 miles, 13 lbs.; 60 miles, 18 lbs. 70 miles, 24 lbs. ; 80 miles, 32 lbs.; and 100 miles, 50 lbs. During the severe storm which passed over London, on February 6, 1867, the anemometer at Lloyd's registered a pressure of 35 lbs. to the square foot-in other words, the wind during that storm acquired a velocity of 83 miles an hour. Wind is most frequently measured by estimation. ; The estimate of the wind's force by the scale o to 12, means that o represents a calm, and 12 a hurricane. If such estimations be divided by 2, and the quotient squared, the result will be the pressure in pounds, approximately. Storms are violent commotions of the atmosphere, occurring in all climates, particularly in the .O NORTH CAPE 29-1 29.3 CHRISTIANSUND HERNOSANDS SHETLAND CHRISTIANIA GIN 29-3 At 8 A.M., November 2, 1863. tropics, and differing from other atmospheric dis- | wind. Numerous attempts have been made to turbances in the extent over which they spread reduce the phenomena of storms to general laws; themselves, their destructive power, and the sudden but it is only quite recently that observations have changes which take place in the direction of the been sufficiently numerous and accurate to furnish the necessary grounds. The foregoing chart of Europe shews, from actual observations made at upwards of 100 localities scattered over that continent, the barometric pressure, and direction and force of the wind, at 8 A.M. of the 2d of November 1863, during part of the course of two storms which passed over Europe at that time. At the same hour of the previous day, the centre of the first storm (I.) was near Christiansund, and that of the second was approaching the west coast of Ireland. The isobarometric lines, or lines shewing where, at the above hour, the height of the barometer was the same, are given for every two-tenths in the difference of the pressure. Hence, where these lines approach near each other, or crowd together, the difference of pressure, or the atmospheric disturbance, was the greatest; and the least where they are most apart-a distinction of the utmost importance in determining where the storm may be expected to rage in greatest fury. The arrows shew the direction of the wind, being represented flying with it. The force of the wind is shewn (1) by plain arrows,, which represent light and moderate winds; (2) by arrows feathered on one side only, →→→, which represent high winds; (3) by arrows feathered on both sides, →→→, which represent strong gales, storms, or hurricanes. Form and Extent of Storm Areas.-The circular isobarometric lines on the chart represent very accurately the general shape storms assume. The area of almost every storm is either circular or slightly elliptical. The outline is occasionally very irregular. The extent over which storms spread themselves is very variable, being seldom less than 600 miles in diameter, but often two or three times that amount, or even more. Direction in which Storms advance.-It may be premised that by the direction of a storm is meant, not the direction of the wind, but the path followed by the centre of disturbance. The direction in which this progressive motion takes place differs in different parts of the world-being determined by the prevailing winds. Thus, about half the storms of Middle and Northern Europe travel from the south-west toward the north-east, and 19 out of every 20, at least, travel toward some point in the quadrant from the north-east to the south-east. Observation shews that the longer axis of the storm is almost always coincident with the direction in which the storm appears to be moving at the time. The storms of the Mediterranean follow a different course. Many of them proceed from the north to the south, influenced probably by the heated air rising from the Sahara. By far the greater number of the storms of North America take their rise in the vast plain which lies immediately to the east of the Rocky Mountains, and thence advance in an eastern direction over the United States; some of them, crossing the Atlantic, burst on the western shores of Europe. But the relation of the American to the European storms is not yet established. The storms of the West Indies generally take their rise from near the region of calms, and tracing out a parabolic course, proceed first towards the north-west, and then turn to the north-east about 30° N. lat., many of them traversing the east coasts of North America as far as Nova Scotia. South of the equator they follow an opposite course, The hurricanes of Hindustan usually pursue a parabolic path, first traversing the eastern coast towards Calcutta, and then turning to the north-west up the valley of the Ganges. The typhoons of the Chinese seas resemble, in the course they take, the hurricanes of the West Indies. Observations are wanting from other parts of the world to determine the course of storms. Everywhere, the course tracked out by storms is determined by the general system of winds which prevail, modified by the unequal distribution of land and water on the surface of the globe. Facts seem at present to point to this general conclusion, viz., Storms follow the course of the atmospheric current in which the condensation of the vapour into the rain which accompanies them takes place. The Rate at which Storms travel, varies from 15 or 17 miles an hour in Europe, to 30 or 40 miles in tropical countries. Relations of Temperature, Rain, and Cloud to Storms.-The temperature increases a few degrees at places toward which and over which the front part of the storm is advancing, and falls at those places over which the front part of the storm has already passed. In other words, the temperature rises as the barometer falls, and falls as the barometer rises. When the barometer has been falling for some time, clouds begin to overspread the sky, and rain to fall at intervals; and as the central depression approaches, the rain becomes more general, heavy, and continuous. After the centre of the storm has passed, or when the barometer has begun to rise, the rain becomes less heavy, falling more in showers than continuously; the clouds break up, and fine weather ushered in with cold breezes ultimately prevails. It should be here remarked, that if the temperature begins to rise soon and markedly after the storm has passed, a second storm may be expected shortly. The rainfall is generally proportioned to the suddenness and extent of the barometric depression at the place where it falls. Direction and Force of the Wind in Storms.— If the winds in Storm II. on the 2d November be attentively_examined, they will be observed whirling round the area of low barometer in a circular manner, and in a direction contrary to the motion of the hands of a watch, with-and be this particularly noted-a constant tendency to turn inwards towards the centre of lowest barometer. The wind in storms neither blows round the centre of lowest pressure in circles, nor does it blow directly towards that centre, but takes a direction nearly intermediate. In other words, the whole atmospheric system flows in upon the centre in a spiral course. This rotatory peculiarity is common to all storms in the northern hemisphere that have yet been examined. In the southern hemisphere, a rotatory motion is also observed round the centre of storms, but it takes place in a contrary direction, or in the direction of the motion of the hands of a watch. Professor Taylor has the merit of having first applied Dove's law of rotation to explain the direction of the rotation of storms round their centre. The cause may be seen by referring to Storm II. on the 2d November. On that morning, the pressure over England being much less than in surrounding countries, if the earth had been at rest, air-currents would have flowed |