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233. Abnormal Distribution of Temperature caused by Currents of the Sea.-The following diagram gives the temperatures of the ten-degree squares as applicable to the North

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From these figures it is evident that if a line be drawn from Cuba in the direction of the Faroe Isles, it will pass through temperatures higher than those which prevail to the eastward or westward of it. Now this anomalous temperature could not be maintained unless there was a general flow of the water of the ocean through the midst of the Atlantic northeastward into the Arctic Ocean. This line of high temperature marks the course of the celebrated Gulf Stream, which, issuing from the Gulf of Mexico, spreads itself north-eastward over the Atlantic. By the warmth it brings from southern latitudes, the British climate is upwards of 20° warmer in winter than it would otherwise be. By the time it arrives at the British Islands, its passage is, no doubt, slow; but from the slow rate at which the temperature of the sea is brought to a complete correspondence with that of the air, it will carry with it enough of its original heat to keep it warmer than the air, though it take some months to traverse the distance from the north of Ireland to Orkney. Observations

prove that the temperature of the sea, as compared with that of the air, is about 3° higher in Orkney than it is in the south of Scotland.

234. Off the Gulf of Guayaquil in South America, the temperature of the sea is only 70°, being from 10° to 12° below the average of tropical seas. This extraordinary depression of temperature is brought about by the cold waters transported thither by the great Humboldt Current from the higher latitudes of the South Pacific. Proceeding westwards along the equator from Peru to the East India Islands, the temperature rises successively to 75°, 80°, 83°, and 84°.5. The last mean temperature, being the highest anywhere on the globe, occurs a little to the east of New Guinea. From this point to the east coast of Africa, including the whole of the northern half of the Indian Ocean, the temperature ranges from 81° to 84°; except near the mouths of some of the large rivers, where it is 1° or 2° less.

235. South of Sierra Leone, the temperature of the Atlantic is only 75°, but from this westwards it rapidly rises to 78°, 80°, 82°, and finally to 83°.5 in the confined waters of the Gulf of Mexico, in the Gulf of Honduras; but as the current traverses the gulf, and mixes with the colder water poured down by the Mississippi, the temperature falls to 76°.

236. The low temperatures on the east coast of North America are caused by the well-known current from the frozen seas of the arctic regions which flows southward on the great bank of Newfoundland.

237. On the west coast of North America, the temperature of the sea increases from east to west. Thus proceeding from the shore westwards, the temperatures of the North Pacific by the ten-degree squares are respectively, between 20° and 30° N. lat., 63°.3, 66°.9, 74°.5, and 77°.7; between 30° and 40° lat., 56.4, 61°.8, 64°.0, 71°.0; and between 40° and 50° lat., 49°.2, 53°.5, and 59°.0. This distribution of temperature is caused by a cold current passing southward along the west coast of North America.

238. On the other hand, the temperature of the South

Atlantic increases westward, showing a flow of the ocean southward along the east coast of South America, thus raising the whole temperature of that coast. The effect of the two opposite currents on the coasts of this continent, is to raise the temperature on the east coast from 8° to 10° above that on the west coast in the same latitude. The southern part of Africa is similarly situated with respect to currents, and the effect on the temperature of the sea is even more marked than in the case of South America,-the temperature in the Gulf of Mozambique being 81°.1, while on the coast of Lower Guinea, in the same latitude, it is only 67°.6, the difference being thus 13.5.

239. The sea on the east of Asia from China northwards becomes warmer as we recede eastward from the continent. Thus between 20° and 30° lat. the temperatures of the squares are 72°.4, 78°.8, and 83°.0; between 30° and 40° lat., 60°.0 and 66°.2; from which northwards in the Sea of Okotsk, it falls to 46°.7 and 34°.5 (?). Thus the east coast of Asia is also chilled by currents descending on it from higher latitudes. Cold currents also set in towards Australia from the southwest, depressing the temperature.

240. The lowest mean temperatures yet observed occur in the Antarctic Ocean, between 60° and 70° S. lat., immediately to the south of the Atlantic Ocean, and Australia and New Zealand. The lowest of these temperatures is 30°.9, occurring in 35° W. long. and 65° S. lat., and lower temperatures no doubt occur in higher latitudes.

241. As regards their influence on climate, ocean-currents raise the temperature of the west of Europe, the east of South America, the east of Africa, and the south of Asia; and depress the temperature on the east and west coasts of North America, the west coast of South America, the west coast of Africa, the east coast of Asia, and the south coast of Australia.

242. It should be here remarked that the information gained from this chart, and from Maury's sailing directions regarding the currents of the sea, is in many cases of a very general character. The service already rendered to navigation

by the knowledge thus disseminated has been of the most substantial kind; for by taking advantage of the currents, distant voyages are now accomplished sooner, and much time and money thereby saved. Much greater would be the advantage if our knowledge of the currents were fuller and more accurate than it is. When the scheme which has been sketched by the British Government, and which is now being worked out by the Meteorological Committee of the Royal Society, shall be completed, we shall be able to know, with sufficient accuracy for all practical purposes, the prevailing winds in different parts of the ocean; and the different currents of the sea, their temperature and density-their origin, course, and termination-the rate of their motion-the limits within which they are bounded-and the annual and secular changes to which they are subject.

243. In some cases the bed of the current is very distinctly marked off. The Gulf Stream, as it issues from the Strait of Florida, is a good example of this; and even after it has turned its course some distance to the eastward, it is in some places sharply defined. Thus when her Majesty's ship Nile was going from Halifax to Bermuda in May 1861, Admiral Sir Alexander Milne found the temperature 70° at the bow, while it was only 40° at the stern, as he entered the Gulf Stream, thus showing a difference of 30° of temperature within the short distance of a ship's length.

244. As an example of what may be expected from a thorough examination of the ocean, we may mention that on a cruise from Gravesend to Farö during August 1865, Dr George Keith, Edinburgh, on the 7th, at 9 P.M., found the temperature of the sea, six miles off Whitby, to be 59°; on the following morning, twenty-five miles off Newcastle, it was only 52°; in the evening, when four miles off the Fern Islands, it was 54°; and on the morning of the 9th, five miles off St Abb's Head, it was 56°, at which point it remained till the latitude of the most northern point of Scotland was passed. Was the low temperature of the sea on the British coast from the Wash to Berwick, caused by a

cold current from the Baltic Sea (where the temperature had been unprecedentedly low during the summer) issuing through the Skager Rack, and thence spreading itself westward toward the opposite coast of England? and if so, was it merely a temporary current, or a permanent current, or a current flowing only during certain seasons of the year? The effect of such a current on the valuable fisheries of the North Sea, and on the climate of the north-east of England, especially when the wind is from the east, must be very great; but so meagre is our knowledge of the temperature of the North Sea, that no answer can be given to the above questions, though the success of the herring-fishings depends to a large extent on the possession of such knowledge.

245. Density of the Sea.-The density of the sea, or the weight of a given quantity of sea-water, at a given temperature, is increased in proportion to the quantity of salt dissolved in it. It is always compared with the density of fresh water, which is considered as unity. The density is ascertained by means of the hydrometer, fig. 17, which is a glass vessel loaded with mercury or shot, and furnished with a scale. The zero point is found by floating it in pure water, at a temperature of 60°, and marking the point on the scale just where it meets the surface of the water.

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Fig. 17.

246. The South Atlantic is a little heavier than the North, their specific gravities being respectively 1.02676 and 1.02664; but the difference between the South and the North Pacific is considerably greater, the South being 1.02658, and the North 1.02548. Hence of the two oceans north of the equator, the Atlantic is heavier than the Pacific; but this difference may be owing in part to the circumstance, that observations are wanting in the middle of the Pacific, where the density is probably greater than in other parts of that ocean, owing to the higher temperature and the absence of currents from the north. Of the three oceans south of the equator, the At

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