the northern hemisphere. On the whole, the results obtained by Dutch vessels confirm the calculations made. by Coffin, from the data collected by Maury.

2. THE UPPER RETURN-TRADE-WIND.

In the preceding section we have seen that the air flows towards the equator from the poles of both hemispheres. In consequence of this tendency it would accumulate in the tropical regions, and its pressure, as indicated by the barometer, would continually increase, were it not that an efflux of the air in the opposite direction takes place in the upper strata of the atmosphere. A fact which bears visible testimony to the existence of this current is the motion of clouds, at a great height, against the Tradewind, which has been observed from the surface of the earth, and expressly mentioned by many navigators. Of these we shall only cite Basil Hall and Paludan. Fendler, at Tovar in Venezuela, has proved the existence of the current numerically by observations carried on for a long series of years. The level at which this return current commences is so high, that it has not been ascertained with certainty by the ascent of the highest peaks of the Cordilleras in the vicinity of the region of Calms; but yet the fact of its existence in that region has been clearly demonstrated. In the night of the 30th of April and 1st of May, 1812, explosions, as if of heavy ordnance, were heard at Barbadoes, so that the garrison of Fort St. Anne was kept under arms. At break of day on the 1st of May, the eastern part of the horizon was clear, while the remainder of the sky was enveloped in a black cloud. The darkness soon extended over the whole sky to such a degree that the place of the windows could not be seen in the apartments, while the trees gave way under the

load of ashes which fell on them. Where did these ashes come from? In the months of April and May the Tradewind is at its height, so that it would have been natural to conclude that they had come from the volcanoes of the Azores. The true source of the ashes was the volcano of Morne Garou, in the island of St. Vincent, which lies 100 miles west from Barbadoes, and from which it is impossible to reach Barbadoes by sea without taking a very circuitous route, in consequence of the direction in which the Trade-wind blows. The ashes from the volcano had been thrown by the violence of the eruption through the under Trade-wind into the upper counter-current. Again, on the 20th of January, 1835, the whole isthmus of Central America was shaken by an earthquake which accompanied the eruption of the volcano of Coseguina, on the Lake of Nicaragua. The violence of the eruption was enormous, so that the sounds were heard at Sta Fé de Bogotá, at a distance of 1,000 miles; while the cloud of ashes was so dense that Union, a seaport town on the west coast of the Bay of Conchagua, was enveloped in total darkness for forty-three hours. Ashes fell also at

Kingston, and at other places in Jamaica, so that the inhabitants were able to learn that the explosions which they had heard had not been those of cannon. These ashes could not have been carried to Jamaica except by the counter-trade-wind, as that island lies to the northeast of Nicaragua.

The fact that ashes from low volcanoes, like Morne Garou and Coseguina, reached the upper current, proves that the eruptions must have been of extraordinary violence. This was the case with Morne Garou, as the eruption to which we have referred formed part of a series of stupendous volcanic phenomena, and marked the finale of the disturbance. In June and July, 1811, Sabrina

Island rose out of the sea, near St. Michael, one of the Azores. The island was raised to a height of 300 feet above the level of the sea, which is 150 feet deep at the place, and it was an English mile in circumference. This event was followed by the continual earthquakes which were felt for months in Arkansas and Ohio, and finally by the total destruction of the city of Caraccas on the 26th of March, 1812. It was not until May that the volcanic energies, which had been so long struggling to get free, burst open the vent of Morne Garou, which had been closed for a century. The noise of the eruption was heard at Rio Apure, which is about as far from the volcano as Naples is from Paris.

Halley was the first to assert the existence of an upper current in the opposite direction to the Trade-wind.* He says, The north-east Trade-wind below will be attended with a south-westerly above, and the south-easterly with a north-westerly above: that this is more than a bare conjecture, the almost instantaneous change of the wind to the opposite point, which is frequently found on passing the limits of the Trade-winds, seems to assure us.'

Halley accordingly considered the south-west wind at the outer limit of the north-east Trade, and the northwest wind at the outer limit of the south-east Trade to be the upper Return-Trade-wind reaching the surface of the earth. The only reason he gives for it is, that it is produced by a kind of circulation.' The phenomenon is, however, a necessary consequence of Halley's theory, who says †, 'The north-east and south-east winds between the tropics must be compensated by as much south-west

6

*An Historical Account of the Trade-winds and Monsoons observable in the Seas between and near the Tropic, with an Attempt to assign the Physical Cause of the said Wind.'- Phil. Trans. 1686, p. 167.

† 'The Cause of the General Trade-wind.'. ·Phil. Trans. 1735, p. 62.

and north-west winds in other parts, and generally all winds from any one quarter must be compensated by a contrary wind somewhere or other; otherwise some change must be produced in the motion of the earth round its axis.'

Leopold von Buch was the first to point out the effect which this descent of the Return-Trade-wind has on the winds of the temperate zone. This he has done in his remarks on the climate of the Canary Islands.*

It is very remarkable and instructive, as well as of the greatest importance for the science of meteorology, to observe the mode in which this NE. Trade is displaced by the SW. winds towards winter. These winds do not commence to the southward and move up northwards, but are felt first on the coast of Portugal, then at Madeira, and afterwards at Teneriffe and the Canaries. They descend as gradually from the upper strata of the atmosphere as they come down from the northward. They had existed continually at this high level, even throughout the summer, when the NE. Trade-wind was blowing with the greatest violence at the sea-level; for the peak of Teneriffe is high enough to reach the current even at midsummer. upper It is hard to find any account of an ascent of the peak in which the strong west wind which had been met with on the summit is not mentioned. Humboldt ascended the peak on the 21st of June; when he reached the edge of the crater he could scarcely keep his feet, such was the violence of the west wind (Relat. i. p. 132). If such a wind had been felt at Santa Cruz or Orotava at that season, the inhabitants would have been quite as much astonished as those of Barbadoes were at the ashes which fell there. I found a similar west wind, although not so high, at the summit of the peak on the 19th of May. George Glass, an attentive and accurate observer, who as a seaman had closely studied the winds of the Canary Islands for many years, says in his

* Physikalische Beschreibung der Canarischen Inseln (Physical Description of the Canary Islands), 1825, p. 67.

"History of the Canary Islands' (p. 251), that 'a strong westerly wind is constantly blowing at the highest points of these islands, during the prevalence of the NE. wind below. This,' he adds, I believe to be the case in all parts of the world where the Trade-winds are felt. I do not venture to explain this phenomenon, but so it is at the top of the Peak of Teneriffe, and on the mountains of some others of these islands.' Glass knew the islands too well not to speak from his own experience on this subject.

These winds descend slowly down the sides of the mountains from the higher strata of the atmosphere. This is clearly to be seen by the clouds from the south, which have enveloped the top of the peak ever since October. They appear lower and lower, and at last rest on the crest of the mountains lying between Orotava and the south coast, which are about 6,000 feet high, and break up there in fearful thunder-storms. Perhaps a week or more elapses after this before they are felt at the sealevel. There they stay for months. Rain falls only on the slopes of the mountains, and the peak covers itself with snow. Can one help believing that the west wind which sailors look for, on the summer voyage from Teneriffe to England, in the neighbourhood and at the level of the Azores, and which they usually find there that the nearly invariable west or southwest wind, which makes sailors call the voyage from New York or Philadelphia to England down-hill, and that from England back again up-hill, is not, as well as the west wind on the summit of the peak, the upper equatorial current, which descends here to the level of the sea? It would follow from this that the upper equatorial current does not reach the pole, at least over the Atlantic Ocean.

We are indebted to Piazzi Smith* for more accurate data as to the boundaries of the two currents. The vertical depth of the under NE. Trade was found to be 9,000 feet. The cloud stratum did not lie, during the preva

* An Astronomical Experiment on the Peak of Teneriffe.'-Phil. Trans. 1859, p. 527.