the southward, between the coasts of Guinea and Brazil, particularly in the months of July and August, notwithstanding the width of the sea is more than 500 lengtes; for the S. E. winds, at that time of the year, commonly extend some degrees beyond their ordinary limits of 4° north latitude, and become more southerly, so as to be sometimes south, or a point or two to the west of south. It then only remains to ply to windward; and if, on the one side, they steer W. S. W., they get a wind more and more easterly; but then there is danger of falling in with the coast or shoals of Brazil; and if they steer E. S. E. they fall into the neighborhood of the coast of Guinea, whence they cannot depart without running easterly as far as the island of St

Themas.

When ships depart from Guinea for Europe, their direct course is northward; bu on this course they cannot go, because, the coast trending nearly east and west, the land is to the northward. Therefore, as the winds on this coast are generally between the south and W. S. W., they are obliged to steer S. S. E. or south, and with these courses they run off the shore; but, in so doing, they always find the wind more and more contrary, so that though, when near the shore, they can lie south, at a great distance they can make no better than S. E., and afterwards E. S. E., with which ourses they generally fetch the island of St. Thomas or Cape Lopez, where finding the wind to the eastward of the south, they sail westerly with it, till, coming to the latitude of 4 degrees south, they find the S. E. wind blowing perpetually.

On account of these general winds, all bound from Europe to the West Indies, or to the southern States of America, consider it most advantageous to get as soon as they can to the southward, so the may be certain of a fair and fresh gale, to run before it to the westward. For the same reason, those bound from the southern States of America to Europe endeavor to gain the latitude of 30 degrees, where they first find the wind begin to be variable, though the most ordinary winds in the North Atlantic Ocean come between the south and west.

And, for the same reasons, those bound to India from America run to the eastward in the variable winds, so as to be in the longitude of 35° or 38° W. when in the latitude of 30° N. From thence they steer south-easterly towards the Cape de Verds, passing 4° or 5° to the westward of them, unless they wish to stop for supplies. Being then in the common route of the European Indiamen, they steer southerly to cross the equator between the longitude of 20° W. and 28° W., where, meeting the S. E. trade winds, they must brace up and sail upon a wind till they get through them and come into the variable winds, where they may steer to the eastward. Near the equator, the trade wind is generally stronger to the westward than to the eastward; and were it not for the fear of falling in with the Brazil coast, a ship might cross the line even farther to the westward. Ships homeward bound, from the Cape of Good Hope towards America, may deviate a little to the westward of their straight course, and cross the equator in the longitude of 30° W., or even as far as 33° W., in order to take advantage of this fresher trade wind.

Between the southern latitudes of 10° and 30° in the Indian Ocean, the general trade winds about S. E. are found to blow, all the year round, in the same manner as in the like latitudes in the South Atlantic Ocean; and during the six months from May to November, these winds reach to within 2 degrees of the equator; but during the other six months, from November to May, a N. W. wind, called the little monsoon, blows in the tract lying between the 3d and 10th degrees of south latitude, in the meridian of the north end of Madagascar, and between the 2d and 12th degrees of south latitude, near the longitude of Sumatra and Java.

In the tract between Sumatra and the African coast, and from 3° of south latitude quite northward to the Asiatic coast, including the Arabian Sea and the Bay of Bengal, the monsoons blow from October to April on the N. E., and from April to October on the S. W. In the former half-year, the wind is more steady and gentle, and the weather clearer, than in the latter six months. In the Red Sea, the winds blow nearly nine months of the year from the southward, that is, from August to May, and the rest of the year from the N. and N. N. W. with land and sea breezes. In the Gulf of Persia, from October to July, the winds blow from the N. W., and about three months from the opposite quarter; these winds being often interrupted by gales from the S. W., and by land breezes.

Between the island of Madagascar and the coast of Africa, and thence northward as far as the equator, there is a tract wherein, from April to October, there is generally a S. S. W. wind, and a contrary wind the rest of the year, with regular land and sea breezes on both coasts.

To the eastward of Sumatra and Malacca, on the north of the equator, and along the coasts of Cambodia and China, quite through the Philippines as far as Japan, the monsoons blow N. E. and S. W., the N. E. setting in about October or November

Between Sumatra and Java to the west, and New Guinea to the east, there are regular monsoons. The N. W. monsoon blows from October to April; the S. E monsoon the rest of the year.

The monsoons do not shift suddenly from one point of the compass to the opposite. In some places the time of the change is attended with calms, in others by variable winds; and it often happens, ou the shores of Coromandel and China, towards the end of the monsoons, that there are most violent storms called ty-foongs, greatly resemling the hurricanes in the West Indies, wherein the wind is so violent, that hardly any thing can resist its force; for this reason, it is more dangerous to approach these shores at the time of the breaking up of the monsoon, than at any other season of he year.

The land and sea breezes prevail principally between the tropics. The sea breeze generally sets in about ten in the forenoon, and continues till about five or six in he evening: at seven the land breeze begins, and continues till about eight in the norning. The cause of these winds is this:-During the day, the sea is not so much heated by the sun as the land, nor so much cooled at night. Hence, in the day time, he cooler air from the sea will rush towards the land, to supply the deficiency occasioned by the greater rarefaction of the air; and from this arises the sea breeze In like manier, during the night, the air at land, being more cooled than that at sea will therefore blow from the land towards the sea, and occasion a land breeze.

A whirlwind is a dangerous phenomenon, caused by the adjacent air rushing in frou all parts towards a centre with great rapidity, and sometimes destroying every objec t passes over in its progressive motion. Water-spouts and whirlwinds arise from the same cause: the latter, being formed at land, are composed principally of air; but the former, being formed at sea, are composed of water.

It was first observed by Dr. Franklin, that the N. E. storms, on the coast of the United States of America, frequently begin earlier in the southern States than in the northern. This he accounts for by supposing a great rarefaction of air in or near the Gulf of Mexico; the air rising thence has its place supplied by the next more uorthern, and therefore denser and heavier air; a successive current is thus formed, to which the coast and inland mountains give a N. E. direction.

Experiments have been made by several persons to determine the velocity of the wind, by observing the space passed over by a cloud or any light substance, and by other methods; and it has been found that the velocity of the wind, in a violent gale. is about 50 or 60 miles per hour.

TIDES.

THE tides are periodical changes of level of the water occurring generally twice in each lunar day. The rise of the tide is known as the flood and its fall as the ebb, the highest rise of any flood being called high tide or high water or full sea, and the lowest fall of any ebb being termed low tide or low water. Each ebb and each flood occupies about six lunar hours. The rise and fall of the tide is the difference of level at low and high water. These periodical changes of level known as the tides should be carefully distinguished from the effects which they produce, known as tidal currents. These refer to the horizontal motion of the water,

The cause of the tides is the unequal attraction of the sun and moon upon different parts of the earth; for they attract the parts of the earth's surface nearer to them with a greater force than they do its centre, and attract the centre more than they do the opposite surface. To restore the equilibrium, the waters take a spheroidal figure, whose longer axis is directed towards the attracting body. The mean force of the sun in raising the tide is to that of the moon only as 1 to 2, for though the mass of the sun is vastly greater than that of the moon, its distance causes it to attract the differentparts of the earth with nearly the same force. A small inland sea, such as the Mediterranean or Baltic, is little subject to tides, because the action of the sun and moon is always nearly equal at the extremities of said seas. The mathematical theory of the tides has not yet reached the point where the tides at any given place, or even the changes from tide to tide at the same place, can be calculated by merely knowing the position of the sun and moon without resort to observation. Nevertheless, by theory combined with observation, we are enabled to predict the tides within moderate limits.

High water occurs on the average of the twenty-eight days, comprising a lunar month, at about the same interval after the time of the moon's crossing (transit over) the meridian. This nearly constant interval, expressed in hours and minutes, is known as the lunitical interval. The observed interval at the time of full and change of the moon at any port is called the establishment of the port, a word which is in common use among navigators, and the amount of which is designated on the charts by Roman numerals and fractions. Thus (vii), near Sandy Hook, on a chart denotes that seven hours and a half after the moon's transit on full and change days high water will occur. The average of all the lunitidal intervals in a month which gives a more correct result, taking one day with another in the course of the month, has been termed by Mr Whewell (one of those who have recently done most for the knowledge of the tides) "corrected establishment," and to distinguish the other number it is called the "vulgar (or common) establishment." In our tables of establishment, the corrected ones are specially marked and are for the ports of the United States, the same with those given upon the Coast Survey charts.

The highest tides do not occur at the precise time of full and new moon, but subsequent to full and change. Upon our Atlantic coast they occur one day after, and on the Atlantic coast of Europe two days after, but on our Pacific coast nearly at full and change. The highest tides are called spring tides, and the lowest, occurring when the moon is near the first and third quarters, are called neap tides. At the periods of fuli and change the attraction of the sun and moon conspire to raise the tide at a given place; at the first and last quarter the high water produced by the moon would occur at the time of the low water caused by the sun, and vice versa, so that the two actions oppose

each other.

By fixing a staff graduated, say, into feet and inches, against the vertical face of a pier or wharf, and observing the mark which the water reaches at low water, we shall see, after some minutes, a slow rise of the water begin, growing more and more rapid for about three hours, then gradually slackening for three more, until, as it nears six hours from the first observation, it again stands for some minutes when it begins to fall towards low water, accelerating as before for three hours, and then slacking off again. The two periods during which the water neither rises nor falls are called the high water stund and low water stand, or sometimes slack water, a term which, to avoid confusion, it is best to apply only to tidal currents. The varying rate of rise and fall of the tide differs much at different places, It is shown at New York and Liverpool in the diagrams, Nos. 1 and 2 on Plate IX. The hours are placed on the horizontal line, and the heights which the water reaches upon the staff on the vertical line.