6 2 Fahr. along the western coast of Scotland and its islands, and rising to 14 5 at Fruholm near the North Cape. And it is also a very significant fact (ascertained by the care ful inquiries of Mr Buchan), that while the summer isotherms cross the British Islands nearly east and west (the tempera ture diminishing pretty regularly from south to north), the winter isotherms traverse them nearly north and south (the temperature diminishing from west to east) ; whilst in Ireland the isotherms seem to envelope the islands in their folds, which increase in warmth from the centre of the island to its sea-board. So in Norway the isothermal lines run parallel to the coast-line, and this alike in summer and in winter, the temperature falling in winter, and rising in summer, with the increase of distance from the sea. Nothing could prove more conclusively than such facts as these (taken in connection with the absence of ice in the harbours of Norway, even as far north as Hammerfest, through the whole winter) the dependence of the mild winter climate of the north-western coasts of Europe upon the proximity of a sea which is warmer than the superincumbent atmosphere ; and we have now to inquire how this great N". E. movement of a stratum of warm water sufficiently thick to retain a surface-temperature con siderably higher than that of the air above it is to be accounted fur. The solution of the problem seems to be afforded by the doctrine of a General Oceanic Circulation, sustained by opposition of temperature only, which was first distinctly propounded in 1845 by Professor Lenz of St Petersburg, on the basis of observations made by him during the second voyage of Kotzebue (1825-1828). Others had been previously led to surmise that "Polar Currents" flow along the floors of the great oceans, even as far as the equator, balancing the superficial counter-currents which are observable in the opposite direction. But Lenz was led to conclude that the whole of the deeper portion of the great ocean-basins in communication with the polar areas is occupied by polar water, which is constantly, though slowly, flowing towards the equator ; whilst conversely the whole upper stratum of equatorial water is as con stantly, though slowly, flowing towards one or both of the poles. And he particularly dwelt on the existence of a belt of water under the equator, colder than that which lies either north or south of it, as an evidence that polar water is there continually rising from beneath towards the surface, a phenomenon which, he considered, admits of no other explanation. He further adduced the low salinity of equatorial water (previously noticed by Hum- boldt, and confirmed by his own observations), compared with that of tropical water, as evidence that the equatorial water of the surface is derived from the polar underflow. And he attributed the maintenance of this circulation to the continually renewed disturbance of equilibrium between the polar and equatorial columns, the greater lateral (because downward) pressure of the former 1 causing a bottom outflow of polar water in the direction of the latter, whilst the reduction of level thus occasioned will produce a surface indraught from the warmer towards the colder areas. The doctrine of Lenz, so far from meeting with the general acceptance to which it had a fair claim, alike on theoretical grounds and from its accordance with the facts ascertained by careful observation, seems to have been put aside and forgotten, a preference being given to the doc trine of the prevalence of a uniform deep-sea temperature of 39, which was supposed to be established by the 1 It must be borne in mind that sea water does not expand like fresh -water in cooling below 39 0- 2, but continues to contract down to its freezing point, which lies between 27 and 25 Fahr., according as it is still or agitated. thermometric observations made in the voyages of D Urvillo and Sir James Ross. No such precaution was taken, however, in these observations as that to which Lenz had recourse, to obviate the effects of the tremendous pressure (1 ton per square inch for every 800 fathoms of depth) to which deep-sea thermometers are exposed ; and it is now certain that the temperatures at great depths recorded by D Urville and Ross were several degrees too high. It was in entire ignorance of the doctrine of Lenz, and under the influence of that of D Urville and Ross, which, had been stamped with the great weight of Sir John Herschel s weight of authority, 2 that Dr Carpenter com menced in 18G8 (in concert with Professor Wyville Thom son) a course of inquiry into the thermal condition of the deep sea, which at once convinced him of the fallacy of the uniform 39 doctrine, and led him to conclusions essentially accordant with those of Lenz. For in the channel of from 500 to GOO fathoms depth between the north of Scotland and the Faroe Islands, they found the deeper half to be occupied by a stratum of glacial water, whose temperature ranged downwards from 32 to 29 0> 5 ; Avhilst the upper half was occupied by a stratum warmer than the normal temperature of the latitudes. This phenomenon was interpreted by Carpenter as indicat ing a deep glacial flow from N.E. to S.W., and a warm upper flow from S.W. to N.E.; and finding that to the west of this channel, on the border of the deep Atlantic basin, the excess of warmth extended to a depth of more than 500 fathoms, he came to the conclusion that the north-moving stratum which brought it could not be an extension of the true Gulf Stream, but must be urged on by some much more general force. A series of tempera ture-soundings taken along the west of Ireland, the Bay of Biscay, and the coast of Portugal, confirmed him in this view, by showing that the division between an upper warm stratum and a cold under-stratum exists in the North Atlantic at a depth of from 700 to 900 fathoms, the whole mass of water below this having either flowed into the basin from the polar area, or having had its temperature brought down to from 39 to 3G 0- 5 by mixture with the polar inflow. And this conclusion was confirmed by the result of temperature-soundings taken at corresponding depths and under the same parallels of latitude in the Mediterranean ; for as they showed a uniform temperature of from 54 to 56, from beneath the stratum of 100 fathoms that was superheated by direct insolation, to the very bottom, it became clear that depth per se could have no effect in reducing the bottom-temperature; and that the cause of the excess of temperature in the mass of water occupying the Mediterranean basin above that of Atlantic water at the same depths, lies in the seclusion of the former from the polar underflow which brings down the deep temperature of the latter. This conclusion having received marked confirmation from temperature-soundings taken in the Eastern seas, was put forward by Carpenter as justify ing the doctrine of a vertical oceanic circulation sustained by opposition of temperature only, quite independent of and distinct from the horizontal circulation produced by wind, which doctrine he expressed in terms closely cor responding with those that had been used by Lenz. And the collection of data for the establishment or confutation of this doctrine was one of the objects of the " Challenger" expedition, which has already made, in the determination of the thermal stratification of the Atlantic between 38 N. lat. and 38 S. lat,, what may be fairly characterised as the grandest single contribution ever yet made to terrestrial physics. The following are the most important of the facts thus 2 See his Physical Geography of the Globe, originally published in the eighth edition of this Encyclopaedia.
