spring of 1839 she died in child-bed, and having shortly after married, his second wife died in 1852. He had two daughters, one of whom only survives. In the domestic circle he showed to great advantage, his gentleness, humour, and piety commending him to old and young. He was a great lover of children, and had the manhood to become a child to children. He was also very fond of dumb animals, and would lay down his book at any time to rollic and play with his dog Topsy. Though subject to fits of dark melancholy and gloom, he always preserved cheerfulness in his household.

Considering the irregular mode of his life, his deep and constant study, his abundant and earnest labours in the Church, and the fact that when he was a child he was very delicate, the wonder is he lived to such a ripe old age. No doubt many reasons might be found for this, chief of which, we think, was the sound and abundant sleep he enjoyed through life. He never lost a night's sleep; he could go to sleep at any time, or almost in any place, and when once fast locked in the arms of Morpheus, it took no ordinary din to rouse him. In his latter years, though his physical vigour rapidly declined, he retained all the buoyancy of spirits and fertility of mind of his youthful days, and till the very last his mouth was filled with the deep things of God's mercy. Having been confined to his room for some time, on the 26th of February, 1870, after hearing his attendant repeat "Rock of Ages," and "There is a Fountain," his ransomed soul passed quietly and peacefully away to be with Christ, which is far better.

It is very unfortunate that—as is the case with so many men of genius-Duncan's worth and usefulness were considerably marred by eccentricities and oddities. Some of the stories told concerning his absence, irregularity, slovenliness, and simplicity are most ludicrous, and excite a pity which is unbecoming toward so great and good a man. No doubt his nature, and the early circumstances by which he was surrounded, led him out in that way; but still, we think, that with effort and watchfulness, his eccentricities and foibles might have been conquered, or, at least, modified, and that it would have been worth an effort is clear from the testimony of those who said his eccentricities made more impression upon them than his discourse. We do not think the Christian minister ought to be unseemly in his dress, manners, or conduct, for it may detract considerably from his usefulness. Despite, however, these failings, we regard Dr. Duncan as one of the greatest men of this age. He did not, perhaps, achieve much in the outer life, but in his experience, as recorded in this volume, he has shewn the paralyzing and degrading influence of unbelief, and the power of the Gospel to save the worst and satisfy the most enquiring, and in this he has not lived in vain.

RORY.

353

ART. VI.-SCIENTIFIC DREDGINGS IN THE
DEEP SEAS.

(Continued from page 233.)

HERE is a unity in the natural sciences which often renders an advance in any one direction a positive gain in collateral subjects. A discovery throws new light upon some obscure department of research; but that light comes not as a single beam, illuminating simply the subject immediately concerned, but as divergent rays, entering other departments, presenting fresh starting points for investigation, and bringing into prominence phenomena more remotely associated with the central subject of inquiry. This has proved to be the case with the recent researches into the physical condition of the deeper parts of the ocean. The few months' work effected in the cruises of the Lightning and Porcupine has given a fresh impulse, not only to the study of the physics of the sea, but has also extended, very materially, our knowledge of Biology, and raised important issues in Geological questions.

A few years ago it was generally thought that, whilst the surface temperature of the ocean varied according to the latitude and the surrounding meteorological conditions-here heated by a tropical sun, and there chilled by Arctic or Antarctic cold-giving rise to the various surface currents and drifts, it was thought that these climatic variations did not penetrate to any great depth, and that below this surface stratum the ocean depths were maintained at, or about, the normal temperature of the crust of the earth. It has now been proved, however, that no such uniformity in the temperature of the deep seas exists, and that their thermal conditions are regulated by influences more potent than those of latitude or the terrestrial surface. Why this remarkable fact was not discovered earlier was probably on account of the defective instruments employed in previous temperature soundings. Many such soundings had been taken, in all latitudes and almost at all depths, but the thermometers had not been protected against the pressure consequent upon the great depth to which they had been lowered; and when such instruments, as had been previously used, were tested by Dr. Carpenter and others, in view of the late researches, the best instruments, under a pressure of three tons to the square inch in a Bramah press, were subject to an error of from eight to ten degrees, whilst those of an inferior character gave no less than from twenty to fifty degrees error. With such a result it was evident that all the work that had been previously done in this direction was little better than useless. But through an ingenious contrivance of Professor Miller, assisted by Mr. Casella, a thermometer has been invented which is almost proof against the effects of pres

sure, and this instrument, known as the "Miller-Casella Thermometer," has hitherto worked so admirably as to be now the one almost solely used in deep sea soundings.

The phenomena requiring explanation in working out the data already obtained in the subject of oceanic temperature, are somewhat numerous and complicated. The general temperature of the deep ocean is found to be considerably below that of the earth's crust. In the same latitude, and at the same depth, a difference of 16 degrees F. has been proved to exist in adjoining masses of water. Thermal conditions are not always regulated by zones of depth, for whilst the colder stratum always underlies the warmer, by reason of its greater density, yet frigid conditions may approach the surface to within 300 fathoms in comparatively low latitudes, as e.g., on the east coast of North America, where the Labrador current flows beneath the Gulf Stream, cooling it to the extent of 20 degrees; and, on the other hand, within the Mediterranean a warmth of 55 degrees F. is maintained at a depth of 2,000 fathoms. Under the Equator, soundings from the Challenger has shown that there exists an intensity of cold equal to 32-4 degrees F., whilst in all the stations immediately north of the Equator, at the same depth, the water was found to be several degrees warmer. Now, what is required is a theory sufficiently comprehensive to embrace all the facts of the case, and explain them on natural principles.

+

It is important to bear in mind, in the first place, that salt and fresh water are very differently affected under the influence of an intense degree of cold; for whilst fresh water increases in density only down to within 7.4 degrees of its freezing point, from which point it gradually expands again until ice is formed; salt water freezes at a lower temperature, and increases in density uninterruptedly down to the freezing point. The consequence of this variation is that, whilst the colder stratum of fresh water, when just about to freeze, floats at the surface, that of sea water will sink and accumulate at the bottom. Now, there can be no question that the cold underflow, almost universally present in the open ocean at great depths, has a polar source. If we examine a map of the North Polar regions, we shall find that the Arctic basin is inclosed by the northern coasts of the European, Asiatic, and American Continents. Behring's Straits communicate with the Pacific Ocean on the one side, and the interval between the coasts of Greenland and the shores of Norway with the Atlantic on the other. But Behring's Straits are so shallow as not to permit of any outflow of Arctic water by that source; and the same must be said of the European seas as far as the Shetlands and Great *The priority of this invention has been disputed and claimed by Messrs. Negretti and Zambra. See discussion in "Nature," vol. ix.

Britain, and also from Faroe to Iceland in mid-channel; the only remaining outlets for the cold and dense water formed under Arctic influences are by the two deep gaps existing in the bed of the ocean between Shetland and Faroe (now known as the "Lightning Channel"), and between Iceland and Greenland on the American side. The Arctic Ocean is thus enclosed in a deep basin with a shallow rim. This rim is notched in the two instances referred to. Under the influence of Polar cold, the surface stratum of water must ever be sinking by reason of its superior density, and will accumulate at the bottom until it reaches the limits of outflow, when it pours its frigid waters into the North Atlantic. It was the good fortune of Professor Thomson and party to hit upon this channel of communication in their first expedition to the North. The current that they discovered and traced in its southerly course probably unites with that which comes from the coasts of Greenland, effecting a union of waters to the south of Iceland, and then, as one vast body, spreads itself like a fan over the temperate and tropical zones. In the "Lightning Channel" this icy current was found to extend from a depth of 600 fathoms, to within 200 fathoms of the surface, and registering a degree of cold at about 30 deg. F. Off the north coast of Ireland it was found at 1,476 fathoms, and at a temperature of 36 deg. F.; and off the shores of Portugal at 1,095 fathoms and 40 deg. F.-thus gradually becoming ameliorated as it travels further and further from its Polar source, absorbing, as it flows, something of the warmth possessed by the temperate water under which it creeps in the lower latitudes.

The causes of this great oceanic movement have now to be considered, but it is not a little perplexing to the unprofessional student to find that between Professor Thomson and Dr. Carpenter (who have been colleagues, working side by side in these investigations) there is a considerable difference of opinion existing as to the primum mobile of these phenomena. Professor Thomson

is the advocate of a theory which makes the cold indraught of Polar water the compensatory return of the Equatorial and Gulf Stream currents. In a lecture on "Deep-Sea Climates," delivered in Queen's College, Belfast, he says in exposition of his view: “The basin of the North Atlantic forms a kind of cul-de-sac ; and while a large portion of the Gulf Stream water, finding no free outlet towards the north-east, turns southwards at the Azores, the remainder, instead of thinning off, has rather a tendency to accumulate in the northern portions of the trough. We accordingly find that it has a depth off the west coast of Ireland of at least 4,800 feet, with an unknown lateral extension. There are no data as yet to determine the rate of the branch of the Gulf Stream which sweeps round the coast of Western Europe and into the Arctic Sea; but it must be very slow, for even so far south as at

latitude 42 deg. N. it has lost all effect upon navigation, its character as a constant current being entirely masked on the surface by the drift of the anti-trades, which has nearly the same direction.

"The Gulf Stream is thus a constant 'river' of hot water, forced into a particular direction by the rotation of the earth, by the constant winds, and by the configuration of the land, and accumulated and modelled by the confined basin of the North Atlantic and Arctic Sea. The cold water which replaces it is supplied under very different conditions.

"Sea water increases steadily in density as the temperature falls, till it reaches its freezing point, about 3 deg. C.; the coldest water, therefore, lies at the bottom, and if over any region warm water be removed by any cause from the surface, as, for instance, in the case of the Equatorial current and the Gulf Stream, its place will be supplied by a general indraught beneath of water from the coldest and heaviest, and consequently usually from the deepest sources from which it can be brought in by gravitation. The cold water is, however, merely drawn in to supply a vacancy, and there is no special reason why it should follow one ingress rather than another. From the low initial velocity of polar water it will tend to flow westwards in passing into lower latitudes, but that tendency will probably be entirely subordinate to specific weight in determining the course of the cold influx and the distribution of layers of water of different temperatures.

"The water of the North Atlantic thus consists, first, of a great sheet of warm water, the general northerly reflux of the Equatorial current, the most marked portion of it passing through the Strait of Florida, and the whole generally called the Gulf Stream, of varying depth, but attaining off the west coast of Ireland and Spain a depth of 800 to 900 fathoms. Secondly, of a general indraught of Antarctic water compensating at all events that part of the Gulf Stream which is deflected southwards; and thirdly, of a comparatively small quantity of Arctic water which, flowing through two or three narrow channels, replaces that portion of the Gulf Stream which makes its way into the Arctic Sea."*

The accumulation of the vast body of warm water, side by side with the cold current, at the entrance of the "Lightning Channel," is explained by the Professor as follows: The North Atlantic forms a basin closed to the northward. Into the corner of this basin, as into a bath—with a north-easterly direction given to it by its initial velocity, as if the supply pipe of the bath were turned so as to give the hot water a definite impulse, this enormous flood is poured, day and night, winter and summer. When the basin is full, and not till then, overcoming its northern impulse, the sur* "Deep-sea Climates," published in "Nature," vol. ii., p. 257.