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the contour of the sea-bed may bring an admixture of frigid water nearer to the surface in particular localities than in the basin generally.

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115. The general results obtained by the correlation of all these data are represented diagrammatically in Diagram V., which, being constructed upon

the same scale as the two preceding figures of the same kind (pp. 457, 458) enables the relation between Depth and Temperature in the Atlantic Basin to be compared with the like relation in the Warm and Cold Areas respectively. For the sake of convenience, the Surface-temperature is here taken at 54°, this (as shown in Table III.) having been its average at those Stations in which the "superheating" did not conspicuously manifest itself.

116. When the rates of decrease of Temperature in successive strata of this deep Atlantic Basin are compared with those which have been shown to exist in the thinner strata of our comparatively shallow Cold area, a very remarkable relation presents itself, the Thermometric changes requiring in the former case a much greater Bathymetric descent than in the latter, but corresponding very closely with them when this allowance is made. This relation may be presented to the mind by ideally extending Diagram II. in a vertical direction, so that its horizontal lines should be separated by four times their interval. It has been shown (§ 98) that in the latter the stratum of about 100 fathoms which lies below the superficial 50 shows but a very slight decrease of temperature, presenting almost exactly the same rate of descent as the stratum between similar depths in the neighbouring Warm area. Now with this 100 fathoms' stratum, a stratum of about 500 fathoms beneath the superficial 100 in the deep Atlantic very closely corresponds, the reduction down to 500 fathoms being at an extremely slow rate. Between 150 and 300 fathoms in the Cold area, however, the rate of reduction becomes very much greater; and this is just what presents itself in the Atlantic Basin between 500 and 1000 fathoms; so that as in the Cold area we come down at very little below 300 fathoms upon a stratum of ice-cold water, so in the Atlantic basin we come down at 1000 fathoms upon a stratum averaging 38°.6. And further, as there is below this a slow progressive diminution of about 2° as we descend through the lower 300 fathoms of the Cold area, so a like progressive diminution is shown as we descend through the lower 1000 fathoms of the deep Atlantic Basin.

117. The significance of these facts becomes yet more apparent, when the varying rates of diminution of temperature in successive strata of the deep Atlantic Basin are reduced to a curve (Diagram VI.), in the same manner as the corresponding rates in successive strata of the Cold area; but with a reduction in the scale of depths in the former case, so as to make 500 fathoms in the deep basin correspond with 150 in the comparatively shallow channel. It is true that there is by no means the same absolute reduction in the one case as in the other; but this difference is just what would be anticipated on the hypothesis we have been advocating. For if it be supposed that the body of ice-cold water brought down from the Arctic basin by the various Polar currents is discharged into the wide and deep Atlantic Basin, it will tend to diffuse itself over its bottom, partly displacing and partly mingling with the water which previously occupied it, so as to form a stratum of considerable thickness, which, while much colder than the

Diagram VI.-Curves constructed from Serial Soundings in the Atlantic Basin.

and 300 fathoms in the Cold area may be taken to indicate that this is the stratum of intermixture between the warm superficial stream coming from the Southward and the deep flow of ice-cold water coming from the Northward, so may the like rapid diminution between 500 and 1000 fathoms in the Atlantic basin be taken as indicating that this is the stratum of intermixture between the great body of surface-water carrying a higher temperature from the Equatorial towards the Polar regions, and the diluted

water nearer the surface, has lost the extreme frigidity which characterizes the current at a comparatively small depth when it comes fresh from the Arctic basin. And just as the rapid descent of temperature between 150

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Polar stream which seems to occupy all the deeper parts of the basin to within about 1000 fathoms of the surface, and thus carries back Polar water to the Equatorial area.

118. These inferences are fully borne out by the Temperature-soundings recently taken by Commander Chimmo, R.N., and Lieut. Johnson, R.N., at various points of the North-Atlantic Basin; for although the temperatures of these Soundings were recorded by unprotected Thermometers, yet the error to which the best of those instruments are subject from the effects of pressure at different depths can now be estimated, and the requisite correction applied to each observation, so as pretty certainly to give the true temperature in each case within a degree. These Soundings give a temperature of about 39° at 1000 fathoms, which is almost exactly accordant with the average of our own; but the "stratum of intermixture," indicated by the rapid reduction of temperature with increase of depth, seems to lie rather nearer the surface, the rapid reduction commencing at about 400 fathoms instead of at about 500. Below 1000 fathoms, at depths progressively increasing to 2270 fathoms, the temperatures are in extraordinarily close accordance with our own, the minimum, however, apparently falling a little lower. Thus at 2270 fathoms, the temperature recorded by an unprotected Casella thermometer was 44°; but the estimated correction for the instrument at that depth being 9°, the real temperature would be 35°.

119. It has thus been shown that the hypothesis advanced in our preceding Report, when worked out in connexion with the peculiar Geographical relations of the Arctic to the North-Atlantic basin, goes far to account for the two orders of phenomena which have now been examined, namely:

(I.) The movement of a vast body of warm water, extending to a depth of several hundred fathoms, in a north-east direction, which moderates the cold of the Boreal area by bringing into it the warmth of that vast expanse of the North-Atlantic Ocean which is heated beneath the Tropical sun.

(II.) The existence of a flow of ice-cold water, at depths greater than 300 fathoms, in a south-west direction along the floor of the channel between the North of Scotland and the Faroe Islands, which contributes, with other frigid streams from the Arctic basin, to diffuse over the NorthAtlantic sea-bed, at depths greater than 1000 fathoms, a Temperature below 39°, ranging downwards with increase of depth to about 35°.

And it further appears :—

(III.) That the "Gulf-stream" may be regarded as a kind of intensification of the ordinary flow of Surface-water from the Equatorial to the Polar area, this intensification being due to the peculiar local conditions which produce an extraordinary "superheating" of water in the Gulf of Mexico, and the diffusion of this superheated water thence over a vast proportion of the North-Atlantic area, raising its Surface-temperature by several degrees.

(IV.) That the Frigid stream which imparts to our Cold area, in the latitude of the Shetland Islands, a Bottom-temperature below 30°, may in like manner be considered as an intensification of the ordinary flow of deep water from the Polar to the Equatorial area, this intensification being due to the peculiar local conditions which limit the flow into the Atlantic basin of the water that has been cooled in the Polar basin, and thus keep it from intermixture with warmer water, whilst, by the narrowing of its channel, it is forced up nearer to the surface.

(V.) That as the temperature of the Gulf-stream is reduced, and the depth of its stratum diminished, the further it diffuses itself over the surface-water of the Atlantic, so the temperature of the Frigid Stream is raised by admixture with the warmer water through which it diffuses itself in the Atlantic basin, whilst it descends deeper and deeper beneath the surface with the increasing depth of the floor on which it rests.

120. It may be questioned, however, whether the low temperature thus shown to prevail, not only over the deepest portion of the North-Atlantic sea-bed, but throughout the enormous mass of water which lies below the "stratum of intermixture" (§ 117), is attributable solely, or even principally, to the cooling effect of the comparatively small quantity of frigid water discharged from the Arctic basin into this vast area, through the narrow channels previously indicated (§ 104). For it is to be remembered that the converse heating-action exerted by the solar rays over the southern portion is continually pumping up this cold water (so to speak) from the depths to the surface; and that this movement will be aided from below by the heat continually imparted from the solid ocean-bed to the colder water which rests upon it. Now as the most trustworthy observations on Deep-sea Temperatures under the Equator, though few in number*, indicate that even there a temperature not much above 32° prevails, it seems probable that part of the cooling effect is due to the extension of a flow of frigid water from the Antarctic area, even to the north of the Tropic of Cancer. It seems impossible to give any other explanation of the low temperatures observed in the 'Hydra' soundings across the Arabian Gulf †, since no frigid water from the Arctic basin could be supposed to find its way to that locality.

121. The unrestricted communication which exists between the Antarctic area and the great Southern Ocean-basins would involve, if the doctrine of a general Oceanic circulation be admitted, (1) a much more considerable interchange of waters between the Atlantic and the Equatorial areas than is possible in the Northern hemisphere; and (2) a reduction in the tem*See Lightning' Report, p. 186.

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+ Lightning' Report, p. 187, note:-The lowest Temperature actually observed in these Soundings, with Thermometers protected on Admiral Fitzroy's plan, was 364°. The temperature of 331° given in the 'Lightning' Report as existing below 1800 fathoms, proves to have been only an estimate formed by Captain Shortland, under the idea that the rate of reduction observed at smaller depths would continue uniform to the bottom, which the Serial soundings of the 'Porcupine' prove to be by no means the case.

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