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lantic is the heaviest, the Indian the lightest, and the Pacific intermediate between the two, the specific gravities being respectively 1.02676, 1.02630, and 1.02658.

247. In the case of land-locked seas, the density falls short of or exceeds the above average specific gravities of the sea, according as the evaporation from their surface falls short of or exceeds the amount of fresh water they receive from rain and the rivers which flow into them. Thus the density of the Mediterranean is in excess, and gradually increases from west to east; the mean of the western half is 1.0286, and of the eastern 1.0291. In the Red Sea the density is also great, and increases from south to north, being in the south 1.0272, and in the north 1.0297. In seas and lakes which have no outlet, and where consequently all the water passes off in evaporation, the greatest degree of saltness is attained;-such are, Tuz Gul Lake in the centre of Asia Minor, 3000 feet above the sea, the saltest of any lake known, containing 32 per cent of saline matters; the Dead Sea, 24 per cent; and the Salt Lake of Utah, 22 per cent.

248. But in seas whose surfaces are small in comparison to the volumes of fresh water poured into them, the density falls far below the average. Thus the average density of the Black Sea is only 1.0143, the highest recorded is 1.0209, and the lowest, at a distance from the mouths of rivers, 1.0114. The average density of the western division of the Baltic Sea is 1.0112, and of the eastern only 1.0042. In the Baltic the highest yet recorded is 1.0232, and the lowest 1.0003, being nearly the density of fresh water.

249. Heavy rains diminish the density. The lowest specific gravity in the Indian Ocean occurs a little south of the equator, whereas in the Atlantic and Pacific Oceans it is to the north of the equator, in the belt of equatorial rains and calms. At the mouth of Loch Fyne, in Argyleshire, Dr William Rankin observed the density of the sea to be 1.0210. This occurred on the 31st August 1860, after a heavy fall of rain, and at high water; next morning it increased to 1.0250, the usual density of the sea at that place. The effect of

heavy tropical rains is often very striking. Thus Dr Ord of H.M.S. Hermes, on the 4th August 1859, at 9 A.M., observed the density to be 1.0266. Heavy rain fell, and in one hour

the density was reduced to 1.0193; in two hours it rose to 1.0253, and in other three hours to 1.0266. In land-locked bays and arms of the sea, particularly if surrounded by hills, the very lowest densities are observed after heavy rains. In Hamnaway Loch, in the Hebrides, Captain Thomas has occasionally taken fresh water from the surface of the sea, the density being 1.0000. Even in mid-ocean, within the tropics, fresh water has been taken from the surface of the sea immediately after torrents of rain had fallen.

250. Hence the chief differences in the specific gravity of the ocean arise from local circumstances. It is high where the evaporation is great, as in the region of the trade-winds; and low where much rain falls, and in high latitudes in the neighbourhood of ice. It is the highest in confined seas where there are few or no rivers and little rain falls; and lowest near the mouths of large rivers, and in seas like the Baltic, which are supplied with large quantities of fresh water. 251. Currents caused by the different Specific Gravities of the Sea. If we except such partial currents as those caused by the tides and the winds, all the currents of the ocean are produced by the different density of the water of the sea at one place as compared with that at another, whether it arises from different degrees of saltness or of temperature. Thus the seas of the two polar basins being about 50° colder than the sea within the tropics, their specific gravity is much greater. To restore the equilibrium, the warmer, and therefore lighter, water of the equatorial regions flows towards the poles, and the colder and denser water of the polar regions towards the equator. If the whole globe was covered with water of the same saltness throughout, the equatorial current would be seen everywhere flowing towards the poles as a surface-current, and the polar current could be detected by soundings flowing everywhere towards the equator as an under-current. But owing to the obstructions offered by the land, and by the inequalities of

the bed of the ocean, and to the different degrees of saltness and therefore of density prevailing in different parts of the sea, these two great currents are broken up into the innumerable currents and counter-currents which diversify the face of the ocean and mark out the highways of commerce.

THE TEMPERATURE OF THE LAND.

252. In countries where the rainfall is pretty evenly distributed among the months, and where snow covers the ground but for a short period of the year, the mean temperature of the soil is almost identical with that of the air. But in countries where the year is divided into wet and dry seasons, and in countries where snow lies a considerable part of the year, the mean annual temperature of the soil may be a little above or a little below that of the air.

253. Influence of Snow on the Mean Temperature of the Soil. -The greatest difference between the temperature of the soil and that of the air occurs when the surface of the ground is covered for some months of the year with snow. Since snow is a bad conductor of heat, it prevents, on the one hand, the propagation of the cold of radiation downwards into the soil, and, on the other, the escape of heat from the soil into the air. Snow thus depresses the temperature of the air in two ways (1) by retaining in the air almost the whole of the cold produced by radiation, and (2) by stopping up the supplies of heat which would otherwise be drawn from the soil. Since, for the same reasons, the temperature of the soil is kept warm, it follows that the temperature of the soil greatly exceeds that of the air when snow lies for some time on the ground. It is in Russia and Siberia that the greatest divergence between the curves of these two temperatures is observed to take place. In Russia, about 120 miles south of Archangel, the mean temperature of the air is 32°, whereas that of the soil is 41°-the soil being thus 9° higher. In Semipolatinsk, in the south-west of Siberia, the temperature of the air is 41°, and of the soil 50°, or 9° higher than that of the air.

254. The daily changes of temperature do not affect the soil to greater depths than three feet. The exact depth varies with the daily range of temperature, by which the amplitude or force of the daily heat-wave is determined, and with the nature of the soil. Similarly the heat of summer and the cold of winter give rise to a larger annual wave of heat propagated downwards, which becomes of less and less amplitude as it recedes from the surface, until it reaches a depth when it ceases to be perceptible. Principal Forbes has shown from the observations made on underground temperature on the Calton Hill, Edinburgh, that the annual variation does not penetrate further than 40 feet below the surface, and that below 25 feet it is very small. The depth at which the annual variation ceases to be observed, and where accordingly the temperature is constant, depends on the conductivity and specific heat of the soil or rocks, and particularly on the difference between the summer and the winter temperature.

255. Owing to the slow rate at which the annual heat-wave is propagated, the highest annual temperature of the trap rocks of the Calton Hill, Edinburgh, at the depth of 24 feet, takes place about the 4th January, and the greatest cold about the 13th July, thus reversing the seasons at that depth. According to Professor J. D. Everett, who has examined the Greenwich observations of Deep-Sunk Thermometers from 1846 to 1859, the highest temperature at a depth of 25.6 feet occurs on the 30th November, and the lowest on the 1st June; and at a depth of 12.8 feet, the highest occurs on the 25th September, and the lowest on the 27th March.

256. From the results arrived at by the observations of the Scottish Meteorological Society, made at depths of 3, 12, 18, 22, 36, and 48 inches below the surface, it has been found that there is a small but steady increase in the mean temperature at these various depths, from 3 inches downwards. To this conclusion there is no exception at any of the stations where such observations have been carried on. Further, Principal Forbes has shown from the Calton Hill observations, that the mean temperature increases from 3 feet down

wards to 24 feet, the latter depth being fully a degree above the former. At 3 feet the mean temperature is 45°.83; at 6 feet, 46°.07; at 12 feet, 46°.36; and at 24 feet, 46°.88.

257. Springs which have their sources at greater depths than that to which the annual variation penetrates, have a constant temperature throughout the year. They may therefore be considered as giving a close approximation to the annual mean temperature of the locality, unless they come from a considerable depth. All experiments made on Artesian wells and other deep borings, prove in the most conclusive manner that the temperature increases with the depth. It has been proved that in the chalk strata forming the lower part of the Paris basin, the temperature increases 1° for every 55 feet. In higher latitudes the increase with the depth is more rapid. Thus at Yakutsk, in Siberia, the temperature at 50 feet is 15.1; at 77 feet, 16°.5; at 120 feet, 21°.0; and at 382 feet, 30°.6-giving an increase of 1° in every 21 feet. The mean rate of increase over the globe is probably 1° of increase of temperature for every 50 English feet of descent.

258. Hence, then, the mean annual temperature increases from the surface as far down into the crust of the earth as man has yet been able to penetrate. It follows from this important result that heat must constantly be passing from the interior of the earth to its surface, whence it escapes into space; and hence the temperature of the whole earth must be cooling from year to year. Sir William Thomson of Glasgow has calculated that during the last 96 million years the rate of increase of temperature under ground has diminished from 1° for every 10 feet, to 1° for every 50 feet, of descent as at present; and adds that, if this action had been going on with any approach to uniformity for 20,000 million years, the amount of heat lost out of the earth would be more than enough to melt a mass of surface-rock equal in bulk to the whole earth; and in 200 million years it would be enough to melt the rocks forming the earth's crust. If this reasoning be just, geologists cannot claim a much higher antiquity for life on the globe than 100 million years.

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