therefore differ 15 degrees in longitude, the sun will pass the meridian of the eastern place 1 hour sooner than the western. The commencement of a lunar eclipse is seen, at the same moment of time, from all places where the eclipse is visible. If then an eclipse of the moon is seen to commence, at one place, at 12 o'clock at night, and at another place, at 1 o'clock; the places differ 15 degrees in longitude, and the last lies eastward of the first. The nautical almanac, published in London, and calculated for the meridian of Greenwich, contains the exact time when the eclipses of the moon commence at that place. When the time of the commencement of an eclipse at any place has been observed, a comparison of it with the time in the almanac will determine the difference of time between the place and Greenwich. If the hour is later than the hour in the almanac, the place is situated to the east of Greenwich; if earlier, to the west. As 1 hour in time is 15 degrees in motion, so is one minute, 15 minutes, and one second, 15 seconds. This would be the easiest and most accurate method of ascertaining the longitude, if we could determine the precise moment of time when a lunar eclipse commences. But this cannot, in general, be determined nearer than 1 minute, and often not nearer then 2 or 3 minutes. A variance of 1 minute would make the difference of 15 minutes or miles in longitude; of 2 minutes, 30 minutes; and of 3 minutes, 45 minutes.

This objection does not lie against the method of ascertaining the longitude by the eclipses of Jupiter's satellites. The telescope enables us to determine the precise moment when they are immersed in the shadow of their primary. The hour at the place, therefore, being ascertained, and compared with the hour in the almanac, we are enabled to determine, as before, the exact difference of longitude.

On the equator a degree of longitude is equal to 60 geographical miles; and of course a minute on the equator is equal to 1 geographical mile. But as all the meridians cut the equator at right angles and approach nearer and nearer till they cross each other at the poles, it is obvious that the degrees of longitude decrease as you go from the equator to the pole.

A TABLE

Showing the number of geographical miles contained in a degree of longitude in each parallel of latitude from the equator.

A map is the representation of some part of the earth's surface, delineated on a plane, according to the laws of projection; for as the earth is of a globular form no part of its spherical surface can be accurately exhibited on a plane.

Maps differ from the globe in the same manner as a picture does from a statue. The globe truly represents the earth; but a map not more than a plane surface represents one that is spherical. But although the earth can never be exhibited exactly by one map, yet by means of several of them, each containing about 10 or

20 degrees of latitude, the representation will not fall very much short of the globe in exactness; because such maps, if joined together, would form a convex surface nearly as round as the globe itself.

Cardinal Points. The upper part of the map is considered as the north; the bottom is south, being opposite to the north; the east is on the right hand, the face being turned to the north; and the west on the left hand, opposite to the east. From the top to the bottom are drawn meridians, or lines of longitude; and from side to side, parallels of latitude. The meridians and parallels are marked with degrees of latitude or longitude, by means of which, and the scale of miles, which is commonly placed in a corner of the map, the situations, distances, &c. of places may be found as on the artificial globe. Thus, to find the distance of two places, suppose Philadelphia and Boston, by the map, we have only to measure the space between them with the compasses, or a piece of thread, and to apply this distance to the scale of miles, which shows that Boston is 286 miles distant in a straight line from Philadelphia. If the places lie directly north or south, east or west, from one another, we have only to observe the degrees on the meridians and parallels, and by reducing these to miles, we obtain the distance without measuring. Rivers are described in maps by black lines, and are wider toward the mouth than toward the head or spring. Mountains are sketched on maps as on a picture. Forests and woods are represented by a kind of shrub; bogs and morasses, by shades; sands and shallows are described by small dots; and roads usually by double lines. Near harbors, the depth of the water is expressed by figures, representing fathoms.

WINDS.

Air is a fine, invisible fluid, surrounding the earth, and extending some miles above its surface; and that collection of it, together with the bodies it contains, circumscribing the earth, is called the atmosphere.

Few natural bodies have been the subject of more experiments than the air; and from these it appears, that it is both heavy and elastic. By its gravity it is capable of supporting all lighter bodies, as, smoke, vapors, odors, &c. And by its elasticity, a small volume of air is capable of expanding itself in such a manner as to fill a very large space, and also of being compressed into a much smaller compass. Cold has the property of compressing air, and heat of expanding it. But as soon as the cause of expansion or compression is removed, it will return to its natural state. Hence, if an alteration be made in any part of the atmosphere, either by heat or cold, the neighboring parts will be put in commotion by the effort which the air always makes to recover its former state.

Wind is nothing more than a stream or current of air, capable of very different degrees of velocity, and generally blowing from one point of the horizon to its opposite. The horizon, like all

sther circles, is divided into 360 degrees; but as these divisions are too minute for common use, it is also divided into 32 equal parts, called rhumbs or points of the compass. Winds are denomi

nated east, west, north, south, &c. according to the points of the compass from which they blow; and with respect to their direction, are distributed into three classes, viz. general, periodical, and variable.

General winds are such as blow always nearly in the same direction. They are found to prevail in the Atlantic and Pacific oceans between the latitudes of about 28 degrees north and south; blowing generally at the equator from the east, on the north side of it between the north and east, and more northerly the nearer the northern limit; and on the south side, between the south and east, and more southerly the nearer the southern limit, and are also called tropical or general trade winds.

Periodical winds are such as blow nearly in certain directions during certain periods of time. The monsoons or shifting trade winds, and the land and sea breezes, are of this kind. The monsoons blow six months in one direction, and then six months in the opposite, the changes happening about the times of the equinoxes, These winds chiefly prevail in some parts of the Indian Ocean. The land and sea breezes are winds, which blow from the land in the night, and from the sea in the day time, changing their direction every 12 hours. They obtain in some degree on the coast of every country, but are most remarkable between the tropics. At the islands between the tropics, the sea breeze begins about nine o'clock in the morning and continues till about nine in the evening; a land breeze then succeeds and continues till about nine the next morning.

The periodical winds arise from the difference in the temperature of the air over land, and of that over water, occasioned by their not acquiring or losing equal degrees of heat in a given time. The Indian ocean is bounded on the east and north by part of Africa, Arabia, Persia, and India, the shores of which are situated within the limits of the trade winds; and the sun, after the vernal equinox, renders the air above these extensive tracts of land hotter than that above the adjacent sea, and thus produces a wind, which soon begins to blow toward the land. This direction of the wind continues from April to October, when the sun having passed to the south side of the equator, the air over the land toward the north becomes colder than that over the water, the direction of the wind is inverted, and it blows on the opposite point the remaining six months of the year. And with respect to the land and sea breezes, the effect of the sun in heating the air over the land in the day time being greater than the heat it produces in the air over the adjacent seas, sea breezes arise; and in the night, the air, which before was hottest, becomes and continues coldest, and a land breeze is the consequence.

Variable winds are those, which are subject to no regularity of duration or change. All the winds in latitudes higher than 40° are of this kind.

Variable, as well as periodical, winds are principally owing, without doubt, to the different temperatures of air incumbent on land and water.

Between the fourth and tenth degrees of north latitude, and between the longitudes of Cape Verd and the easternmost of the Cape de Verd Islands, is a tract of sea, which seems to be condemned to perpetual calms, attended with dreadful thunder and lightning, and such frequent rains, that it has acquired the name of the Rains. This phenomenon seems to be caused by the great rarefaction of the air on the neighboring coast, which causes a perpetual current of air to set in from the westward, and this current meeting here with the general trade wind, the two currents balance each other, and cause a general calm; while the vapors carried thither by each wind, meeting and condensing, occasion these frequent deluges of rain.

Dr. Derham, from repeated observations upon the motion of light, downy feathers, found that the greatest velocity of the wind was not above 60 miles in an hour. But Mr. Bruce justly observes, that such experiments must be subject to great inaccuracy, as the feathers cannot proceed in a straight line; he therefore estimates the velocity of winds by means of the shadow of a cloud over the earth, by which he found, that, in a great storm, the wind moves 63 miles an hour; in a fresh gale, 21 miles an hour; and in a small breeze, 10 miles an hour. Mr. Rouse makes the velocity of a hurricane 100 miles an hour.

TIDES.

By the term tide is meant the regular alternate rising and falling of the water in the sea and rivers. The phenomena of the tides occasioned a variety of opinions among the ancient philosophers, and the cause was considered as one of the greatest mysteries in nature. It remained in obscurity till the latter end of the 16th century, when Sir Isaac Newton clearly pointed it out, and showed the agreement of its effects with the observed phenomena.

A heavy body, being thrown up in the air, falls again to the earth in a direction perpendicular to its surface, or in a line tending to its centre. The cause of the body's falling is a species of attraction, called gravity or gravitation. This principle operates not only between the earth and all bodies near its surface, but also between all the bodies which compose the solar system, and probably between all the bodies and systems of the universe. And it is abundantly proved by experiment and observation, that the force of gravity is inversely as the squares of the distances of the bodies from one another, that is, the force decreases in the same ratio as the squares of the distances increase, and vice versa.

The flowing and ebbing of the sea are to be attributed to the attraction of the sun and moon; but principally to that of the moon on account of its less distance from the earth.

The attractive force of the moon varies at different distances,