wind resumes its natural course of S. E. These winds are called the N.W. and S. E. monsoons.

The monsoons do not change suddenly from one point of the compass to the opposite; between the expiration of one, and the commencement of the other, the winds are light and variable, and sometimes calms prevail, until the regular monsoon commences, and acquires sufficient strength to blow steady.

The shifting of the N. E. and S.W. monsoons is frequently attended with violent squalls; for which reason ships between the coasts of Malabar and Africa, if bound to Bombay from the southward, never attempt to make the former coast at the breaking up of the N. E. monsoon, particularly in the month of May; hence, likewise, they avoid the Coast of Coromandel in the month of October: for it is a fact worthy of remark, that the bad weather month on the Coast of Malabar is the fine weather month on the Coast of Coromandel, and vice versâ, although these coasts are situate on the same peninsula.

The most obvious cause of the above periodical changes in the wind appears to be the situation of the sun in the ecliptic at the different seasons of the year; for when the sun approaches the Tropic of Cancer, the soil of Persia, Bengal, China, and the adjoining countries, becomes so much more heated than the sea to the southward of those countries, that the current of the general N. E. trade-wind is interrupted, so as to blow at that season from the south to the north, contrary to what it would do if no land were there; but as the high mountains of Africa, during all the year, are extremely cold, the low countries of India to the eastward of it becomes hotter than Africa in summer, and the air is naturally drawn thence to the eastward: hence it is that the wind in those parts blows from the S.W. between April and October, contrary to the trade-winds in the Atlantic and Pacific Oceans in the same latitudes; but when the sun retires towards the Tropic of Capricorn, these northern parts become cooler, and the general trade-wind assumes its natural direction from the N.E.

Upon the same principle we account for the monsoons adjoining New Holland, which we find is an immense tract of land to the S. E. of the Sunda and Molucca Islands; for when the sun is in the Tropic of Cancer, the current of air, even independent of the trade-wind, will move from the S. E., to restore the equilibrium to the N. W.; on the contrary, in the months of November, December, and January, whilst the sun is nearly vertical over a great part of New Holland, the current of air, through the Sunda and Molucca Islands, will come from the N. W., to fill up the vacuum made by the rarefication, and thus occasion an alternate S. E. and N.W.

monsoon.

The cause of land and sea breezes, which prevail principally between the Tropics, and never extend above three or four leagues from the shore, may be explained after the same manner as the monsoons. For during the day the sea is not so much heated by the presence of the sun as the land, nor is it so much cooled during the night; therefore, when the earth begins to be violently heated in the course of the day, the cooler air from the sea will

rarefaction of the air; and hence arise the sea-breezes. On the other hand, the land becoming cooler than the water in the absence of the sun, the current of air, a few hours after sunset, flows from the land to the sea, and thus produces the land-breeze.

TIDES.

A TIDE is that regular motion of the waters of the ocean by which they rise and fall in certain intervals of time. The rising of the water is called the flux, or flood; and its falling, the reflux, or ebb. When the water has attained its greatest height, it is said to be high water; and when it is done falling, it is called low water.

These periodical motions of the waters are effected by the unequal attraction of the sun and moon, but chiefly that of the latter object, on the different parts of the earth. For the discovery of the laws by which this general principle of attraction is governed, we are indebted to the great Sir Isaac Newton, who has demonstrated that the power of attraction diminishes as the distance increases, in proportion to the squares of those distances.

Now it is evident by the above law, that those parts of the earth nearest the moon, are more attracted by her than the central parts; and that the central parts will be more attracted than those which are farthest from her: and therefore the distance between the earth's centre and the waters upon its surface, under and opposite to the moon, will be increased; so that if the earth's surface were covered with water, it would assume a spheroidal, or egg-like figure, the longest diameter of which would be directed to the moon's centre. Hence those parts of the earth directly under and opposite the moon, that is, where the moon is in the zenith and nadir, will have the flood, or high water, at the same time; while those parts at 90 degrees distance, or where the moon appears in the horizon, will have the ebb, or lowest water, at that time. As the moon apparently shifts her position from east to west, in going round the earth every day, the longest diameter of the spheroid following her motion, occasions two floods and ebbs, in about every 24 hours and 49 minutes, which is the length of a lunar day; or the interval between the moon's passing the meridian of any place, and returning to the

same.

The earth's diameter bears a considerable proportion to its distance from the moon, but is next to nothing when compared to its distance from the sun; therefore the difference of the sun's attraction on the sides of the earth under and opposite to him, is much less than the difference of the moon's attraction on the sides of the earth under and opposite to her, and consequently the moon must raise the tides much higher than they can be raised by the sun.

From this theory, it may be thought the tides ought to be highest directly under and opposite the moon; but we find that in open seas.

where the water flows freely, the moon is generally past the meridian when it is high water. The reason is obvious; for though the moon's attraction were to cease altogether, when she was past the meridian, yet the motion of ascent communicated to the water before that time, would make it continue to rise for some time after; much more must it do so when the attraction is only something diminished.

The times of high water do not always answer to the same distance of the moon from the meridian at the same places; but are variously affected by the action of the sun, which brings them on sooner when the moon is in her first and third quarters, and keeps them back later when she is in her second and fourth; because, in the former case, the tide raised by the sun alone, would be earlier than the tide raised by the moon; and in the latter case, later.

When the moon is in perigee, or at her nearest distance from the earth, she attracts strongest, and therefore raises the tides most; the contrary happens when she is in apogee, or at her greatest distance from the earth, because of her weaker attraction. At new moon, when the moon is in conjunction with the sun, the tides are raised by the joint attraction of both luminaries, and therefore will be highest; the same is the case at full moon, when the sun and moon are in opposition: for whilst the moon raises the tides under and opposite her, the sun acting in the same line, raises the tides under and opposite to him, whence their conjoint effect is the same as at the change, and in both cases occasions what are called spring-tides. But at the quarters, the sun raises the tides where the moon depresses them, and depresses them where they would be raised by the moon: hence it is the difference of their actions that produces the tides at the quarters, and these are called neap-tides. But these tides do not happen till a day or two after the above times; because in this, as in other cases, the effect is not greatest or least when the immediate influence of the cause is greatest or least, but some time afterward.

The sun being nearer the earth at the beginning than at any other time of the year, its attraction will then be most powerful; and of course about January the spring-tides will be greater than at any other time, and greatest of all if the moon at the same time should happen to be in perigee.

When the moon is in the equinoctial, the tides are equally high in both parts of the lunar day; but as the moon declines from the equinoctial towards either Pole, the tides are alternately higher and lower at places having north or south latitude. Whilst the moon has north declination, the greatest tides in the northern hemisphere are when she is above the horizon, and the reverse whilst her declination is south.

The tides rise higher at any place in proportion as the moon is nearer to the zenith or nadir of that place at the time of her passing the meridian ; because the action of the moon is there strongest: hence the tides are greater between the Tropics than at any other parts, and less near the Poles.

All the above particulars would exactly obtain were the whole surface of the earth covered with deep water; but since there are multitudes of islands besides continents lying in the way of the tide, which interrupt its direct

particular solutions, wherein the situation of the shores, straits, shoals, winds, and other things must be considered. For instance, as the sea has no known passage between Europe and Africa, let them be supposed one continent, extending from Weigate Straits, in latitude 78° North, to the Cape of Good Hope, in latitude 34° South; the middle of these two would be in about 19° North, near Cape Blanco on the west coast of Africa. But it is impossible the flood-tide should set to the westward upon the western coast of Africa (for the general tide, following the course of the moon, must set from east to west), because the continent for above 50 degrees, both northward and southward, bounds that sea on the east; therefore, if any regular tide, proceeding from the motion of the sea, from east to west, should reach this place, it must come either from the north of Europe southward, or from the south of Africa northward.

This opinion is further corroborated, or rather fully confirmed, by common experience, which shews that the flood sets to the southward along the west coast of Norway, from the North Cape to the Naze, or entrance of the Baltic Sea, and so proceeds to the southward along the east coast of Great Britain, and in its passage supplies all those ports which lie in its way, one after another. The coast of Scotland has the tide first, because it comes from the northward to the southward. On the full and change days it is high water at Aberdeen at 1h. 11m., but at Tynemouth Bar not till 2h. 50 m.; rising thence to the southward, it makes high water at the Spurn 20m. after 5h.; at Yarmouth Roads 40 m. after 8h.; at Harwich 11h. 30 m.; at the Nore Light 9m. after 1 h., and at London Bridge at 2h. 7m., all in the same day. And although this may seem to contradict the hypothesis of the natural motion of the tides being from east to west, yet as no tide can come west from the main continent of Norway or Holland, it is evident the tide we have been tracing, by its several stages from Scotland to London, is supplied by that tide, the original motion of which is from east to west. As water always inclines to its level, it will in its passage fall to any other point of the compass, to fill up vacancies where it finds them; and yet not contradict, but rather confirm the hypothesis.

From these circumstances it is evident, that the direct course of the rising tides from east to west being interrupted by the land lying in their way, they are often obliged to make a long circuit, and to flow in various directions; whence the setting of the tides, and the times of high water, are different at different places.

Lakes and inland seas, such as the Caspian Sea, the Mediterranean, and the Baltic Seas, have little or no sensible tides; for they are usually so small, that the attractive influence of the sun and moon is nearly equal at both extremities, and cannot therefore sensibly affect the water.

When the time of high water at any place is mentioned generally, it is to be understood of the time when it is high water at that place on the day of new or full moon; or the time past noon when it is high water on the day on which the sun and moon are together on the meridian of the place. Among pilots it is customary to reckon the time of flood, or high water, by the point of the compass the moon is supposed to bear on at that time, allowing three-quarters of an hour for each point. In places,

for instance, where it is flood at noon on the days of full and change, the tide is said to flow north and south, or at 12 o'clock. In places where the moon is supposed to bear 1, 2, 3, 4, or more points to the east or west of the meridian, when it is high water on the same day, the tide is said to flow on such a point; so if the moon is supposed to bear S. E. at flood, it is said to flow Ŝ. E. and N. W., or 3 hours before the moon comes to the meridian, that is, at 9 o'clock; if she bears S. W., it flows S. W. and N. E., or at 3 hours after the southing; and in like manner for other points of the moon's bearing. But this absurd custom of reckoning the tides by the bearing of the moon, should be exploded, as founded in error; for the moon takes a greater or less portion of time in passing over any given number of points of the compass.

In some places it is high water on the shore, or by the ground, while the tide continues to flow in the stream or offing; and according to the length of time it flows longer in the stream than on the shore, it is said to flow tide, and such part of tide, allowing 6 hours to a tide. Thus 3 hours longer in the offing than on the shore make tide and half-tide; an hour and a half longer make tide and quarter-tide; three-quarters of an hour longer make tide and half-quarter-tide, &c.

The common method of finding the time of high water at any place is contained in the following particulars.

OF LEAP YEAR.

The length of the solar year being nearly 365 days 6 hours, and the common year containing only 365 days, one day is added every fourth year to the month of February, making that year contain 366 days, which is called bissextile, or leap year, and is found as follows:

To find the Leap Year.

RULE. Divide the given year by 4, and if there be no remainder, it is leap year; but if 1, 2, or 3 remain, they shew that it is so many years after leap year.

EXAMPLE. The year 1846, divided by 4, gives 461, and the remainder 2, which shews that it is the second year after leap year.

OF THE EPACT.

The Epact is the moon's age at the beginning of the year: it increases* 11 every year, being the excess of the solar year of 365 days, above the lunar year of 354 days, or 12 lunations. It is also observed, that the moon goes through all her variety of aspects, with respect to the sun, in the course of 19 years; so that at the end of that period, which is called the lunar cycle, the new and full moons return on the same days of the month, and nearly at the same hours. Hence the following Rule:

To find the Epact.

RULE. Divide the given year by 19; multiply the remainder by 11, and the product will be the Epact, if it does not exceed 29; but if it does, divide