WE have already briefly described the tides. We go on to observe that

the fluctuation of the tides produces another and more constant rotation of the waters of the ocean, from east to west, in this respect following the course of the moon. This may be considered as one great and general current of the waters of the sea; and although it be not every where perceptible, it is nevertheless every where existent, except when opposed by some particular current or eddy, produced by partial and local causes. This tendency of the sea towards the west is plainly perceivable in all the great straits of the ocean; as, for instance, in those of Magellan, where the tide, running in from the east, rises twenty feet high, and continues flowing six hours; whereas the ebb continues but two hours, and the current is directed to the west. This proves that the flux is not equal to the reflux; and that from both results a motion of the sea westward, which is more powerful during the time of the flux than the reflux.

This motion westward has been sensibly observed by navigators in their passage back from India to Madagascar, and so on to Africa. In the great Pacific Ocean, also, it is very perceivable: but the places where it is most obvious are those straits which join one sea to another. In the straits between the Maldivia Islands; in the gulph of Mexico between Cuba, and Jucatan. In the straits of Paria, the motion is so violent, that it has received the name of the Dragon's Mouth. It is observed northward also, in the sea of Canada, in Waigat's straits, in the straits of Java, and in short, in every strait where the ocean on one part pours into the ocean on the other. In this manner therefore is the sea carried with an unceasing circulation round the globe; and at the same time that its VOL. IV.

I i

1

waters are pushed back and forward with the tide, they have thus a progressive current to the west, which though less observable, is not the

less real..

But besides this general motion of the sea from east to west, there are many other currents which are confined to particular parts of it. These are found to run in all directions, east, west, north, and south; being formed by various causes; the prominence of the shores, the narrowness of the straits, the variations of the wind, and the inequalities of the bottom. These are of the greatest consequence to the mariner, and a knowledge of them is absolutely necessary to their safety and success. It has often happened, that when a ship has unknownly got into one of these, every thing seems to go forward with success, the mariners suppose themselves every hour approaching their wished for port, the wind fills their sails, and the ship's prow seems to divide the waters; but at last, by sad experience, they find, that instead of going forward, they have been all the time receding. Currents, therefore, make a considerable article in the business of navigation; and the direction of their streams, and their rapidity have been carefully noticed. This has been done in different manners; as, by observation on the surface of the current; or by the driving of the froth along the shore; or by throwing out what is called the log-line, with a buoy made for that purpose, and by the direction and motion of this, they judge of the motion, and the rapidity of the current.

The setting of a current, or its progressive motion, may be either quite down to the bottom, or to a certain determinate depth. As the knowledge of their direction and velocity is of great importance in navigation, it is highly necessary to discover both in order to ascertain the ship's situation and course with as much accuracy as possible. The most successful method which has hitherto been practised by mariners for this purpose is as follows. A common iron pot, which may contain four or five gallons, is suspended by a small rope fastened to its ears or handles, so as to hang directly upright, as when placed upon the fire. This rope, which may be from seventy to an hundred fathoms in length, being prepared for the purpose, is coiled in the boat, which is hoisted out of the ship at a proper opportunity, when there is little or no wind to ruffle the surface of the sea. The pot being then thrown overboard into the water, and immediately sinking, the line is slackened till about seventy or eighty fathoms of it are run out; after which it is fastened to the stern of the boat, by which she is restrained, and resides as at an anchor. The velocity of the current is then easily tried by the log and half-minute glass, the usual method of discovering a ship sailing at sea. The course of the stream is next determined by the compass provided for that operation. Having thus found the setting and drifting of the current, it remains to apply the whole to the purposes of navigation. 1. to Currents are generally found to be most violent under the equator, where, indeed, all the motions of the ocean are most perceivable. Along the coast of Guinea, if a ship happens to overshoot the mouth of any river it is bound to, the current prevents its return; so that it is obliged to steer out to sea, and take a very large compass, in order to correct the,

former mistake The currents here set in a direction contrary to the general motion of the sea westward; and so great is their force, that a passage which, with the current, is made in two days, is with difficulty performed in six weeks against t. However, they do not extend above twently leagues from the coast; and ships going to the East Indies, take care not to come within the sphere of their action.

At Sumatra the currents, which are extremely rapid, run from south to north: there are also strong currents between Madagascar and the Cape of Good Hope. On the western coasts of America, the current always runs from the south to the north; a south wind continually blows there, and most probably occasions this appearance.

But the currents that are the most remarkable are those in the Mediterranean sea. From the Atlantic ocean through the straits of Gibraltar, and from the Euxine sea by the Archipelago, there are perpetual currents flowing into it. This is one of the most extraordinary appearances in nature. This sea continually receives not only these currents, but also a very great influx from a multitude of rivers, among which are the Tyber, the Po, the Rhone and the Nile: outlets there are none visible; no straits, no rivers, but such as bring it fresh supplies; and yet its waters do not increase: they are no higher than they were a thousand years ago.

Mankind, in every age, have enquired how this vast quantity of waters has been disposed of, and how this sea, which is always receiving and never returning, has no sensible increase of its waters. The learned Kircher accounts for this by subterraneous passages into the Red Sea, by which he supposes the waters to be conveyed from the Mediterranean. Others have had recourse to evaporation by the heat of the sun, by which they suppose the superfluous waters are drained off. To support this idea, they have entered into long calculations upon the extent of its surface, and the quantity of water that would be raised from such a surface in a year. They then compute how much water runs into it, by its rivers and straits in that time; and they find the quantity raised by evaporation, to exceed the quantity received by rivers and straits. But these curious philosophers seem to have forgotten that the Euxine sea, and the ocean at large evaporate as well as the Mediterranean: as these are subject to the same drain, it follows, therefore, that all the seas must in this respect be upon a par; and, consequently, there must be some other cause for this appearance.

The most satisfactory account seems to be that there is an under current in the straits of Gibraltar that carries out as much water as is sufficient to keep an equilibrium in the waters.

The existence of under currents is evident at many places. Thus in the English channel, between the north and south Foreland, the water runs tide and half-tide, that it, it is either ebb or flood in that part of the Downs three hours before it is so off at sea: a certain sign, that though the tide of flood runs aloft, yet the tide of ebb at the same time runs under-foot, that is, close by the ground, and so, at the tide of ebb above, it will flow under-foot.

These under currents run also in the Baltic sound, of which an able seaman gave to Dr. Smith the following proof. That being there with one of the King's frigates, they went with their pinnace into the mid-stream, and were violently carried by the current. Soon after that they sunk a b. sket with a large cannon bullet to a certain depth of water, which gave a check to the boat's motion; and then sinking it still lower and lower by degrees, the boat was at last driven a-head to the windward against the upper current, the current aloft not being above four or five fathom deep. He added that the lower the basket was let down the stronger the under current was found. The same experiment has since been made in the straits of Gibraltar, with the same appearances.

From this principle, it is easy to account for that continual indraught out of the Atlantic to the Mediterranean, through the straits of Gibraltar, which is a passage about twenty miles broad; yet without any sensible rising of the water along the coasts of Barbary, or any overflowing of the lands, which nevertheless lie there very low. Dr. Halley's solution, therefore, by evaporation, falls to the ground.

There are many shifting currents which do not last, but return at certain periods. These, most of them, depend upon and follow the anniversary winds, or monsoons, which, blowing with great violence in one place, cause a current in another. Thus in the straits of Sunda, when the monsoons blow from the west, in the month of May, the currents set to the eastward, contrary to the general motion. Between the island of Ceylon and Madura, when the western monsoons set in, in December, or when the winds blow from the north-west, or between the north and north-west, the currents then set to the south-east, or between the south and south-east.

In the Bay of Sans Bras, not far from the Cape of Good Hope, there is a remarkable current, which runs from east to west, to the land-ward ; and this the more vehemently as it is opposed by winds from a contrary direction. The cause, probably, is owing to some adjacent shore which is higher than this.

The number of the currents at sea is impossible to be recounted, nor can they indeed be fully known; new ones are daily produced by a variety of causes, and as quickly disappear. When a regular current is opposed by another in a narrow strait, or where the bottom of the sea is very uneven, a whirlpool is often formed. These were, in ancient days, considered as the greatest obstructions to navigation, and the ancient poets and historians speak of them with terror; they are described as swallowing up ships, and dashing them against the rocks at the bottom: apprehension did not fail to add imaginary terrors to the description, and placed at the centre of the whirlpool a dreadful den, fraught with monsters, whose howlings served to add new horrors to the dashings of the deep. Mankind at present, however, view these eddies of the sea with very little apprehension; and some have wondered how the ancients could so much overcharge their descriptions. But this is not difficult to account for. In those times when navigation was as yet in its infancy, the slightest concussion of the waves generally sent

the poor adventurer to the bottom: is it then to be wondered at that he was terrified at the violent agitation of a whirlpool? When his little ship, but ill fitted to oppose the fury of the sea, was got within the vortex, there was no possibility of ever returning. To add to the fatality, they were always near the shore; and along the shore was the only place where this ill provided mariner durst venture to sail. Whirlpools were therefore dreadful impediments to him; for if he attempted to pass between them and the shore, he was liable to be sucked in by the eddy; and if to avoid them he stretched out to sea, he was often sunk by the storm. But in our improved state of navigation, Charybdis and the Eripus, with all the other irregular currents of the Mediterranean, are no longer formidable. Mr. Addison, not attending to this train of thinking, upon passing through the straits of Sicily, was surprised at the little there was of terror in the present appearance of Scylla and Charybdis; and seems to be of opinion, that their agitations are much diminished since the times of antiquity. In fact, from the above reasons, all the wonders of the Mediterranean are described in much higher colours than they merit, to us who are acquainted with the more magnificent terrors of the ocean. The Mediterranean is one of the smoothest and most gentle seas in the world; its tides are scarce perceivable, except in the gulph of Venice, and shipwrecks are less known there than in any part of the world.

In the ocean, indeed, where the currents are violent, and the tempests fierce, whirlpools are particularly dangerous That called the Maelstroom upon the coast of Norway, is considered as the most dreadful and voracious in the world. The name signifies the navel of the sea, because the natives suppose that a great part of the water of the sea is sucked up, and discharged again by its vortex. A description of its internal parts is not to be expected, since none who ever were there have returned to bring us information. The body of the waters which form this whirlpool are extended in a circle about thirteen miles in circumference. In the midst of this stands a rock, against which the tide in its ebb is dashed with inconceivable fury. The noise of it is heard several leagues off, and the vortices and pits are of such an extent and depth, that if a ship comes within its attraction, it is inevitably absorbed and carrried down to the bottom. No skill in the mariner, nor strength of rowing, can work an escape; the sailor at the helm finds the ship first going in a current opposite to his intentions; the motion of the vessel, though slow in the beginning, becomes every moment more rapid; it goes round in circles still narrower and narrower, till at last it is dashed against the rocks, and instantly disappears : nor is it seen again for six hours; and then the tide flowing, the remnants are vomitted forth again with the same violence with which it was drawn in.

t

The intervals of tranquility are only at the turn of the ebb and flood, and last but a quarter of an hour, its violence then gradually returns. When the stream is most boisterous, and its fury heightened by a storm, it is dangerous to come within a Norway mile of it; boats, yachts, and ships having been carried away by it. It likewise happens frequently that whales come too near the stream, and are overpowered by its