MAPS, AND THEIR USE.

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 carth 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 statuc. 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 north is considered as the upper part of the map; the south is at the bottom, 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 outmost of 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 me ridians 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 fathams.

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 thesc 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 other 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 denominated east, west, north, south, &c. according to the points of the compass on 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. 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.

At

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 be gins 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 in 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 seas 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, being greater at a small distance and smaller at a great distance. Its power is found to diminish as the squares of the distances increase. Thus, if at the distance of 10,000 miles, the attractive force be considered as 4, at the distance of 20,000 it will be only 1. Hence the waters on the side of the earth directly under the moon are more attracted by the moon than the central parts of the earth, because they are nearer to the moon, and the central parts of the earth are more attracted than the waters on the opposite side of the earth. Consequently the waters directly under the moon will be as it were attracted from the centre of the earth and be made to rise towards the moon; and the centre of the earth will be as it were attracted from the waters on the side of the earth opposite to the moon, so that those waters will be less near the earth's centre than if the moon did not operate, i. e. they will risc. On the meridian directly under the moon, therefore, there will be a high tide and a similar one on the opposite side of the earth, at the distance of 180°. On each side, however, at 90° distance from that meridian, in consequence of the moon's very oblique attraction, the waters will be depressed.

The tides are higher than ordinary twice a month, viz. about the times of the new and full moon; and these are called spring tides. Because at these times the attraction of the sun conspires with that of the moon, or their agency is in the same right line; and consequently the tides must be more elevated. When the two luminaries are in conjunction, or on the same side of the earth, they both conspire to raise the water on the nearest and remotest part; and when they are in opposition, that is, when the earth is between them, the part nearest to the one is remotest from the other, and vice versa, consequently the effects of their agency are united. The tides are less than ordinary twice a month; that is, about the times of the first and last quarters of the moon; and these are called neap tides. For in the quarters of the moon, the sun raises the water where the moon depresses it; and depresses it where the moon raises it; the tides are made therefore by the difference of their actions.

LENGTH OF MILES IN DIFFERENT COUNTRIES.

There is scarcely a greater variety in any thing than in this sort of measure; not only those of separate countries differ, as the French from the English, but those of the same country vary in the different provinces, and all commonly from the standard. Thus the common English mile differs from the statute mile, and the French have three sorts of leagues.

We shall here give the miles of several countries, compared with the English, by Dr. Hally.

The English statute mile consists of 5280 feet, 1760 yards, or & furlongs.

Eleven Irish miles are equal to fourteen English.

The Russian verst or werst is little more than English.

The Turkish, Italian, and old Roman less, mile is nearly 1 En

glish.

The Arabian, ancient and modern, is about 14 English.

The Scotch mile is about 14 English.

The Indian is almost 3 English.

The Dutch, Spanish, and Polish, is about 34 English.

The German is more than 4 English.

The Swedish, Danish, and Hungarian is from 5 to 6 English, The French common marine league is nearly 3, and

The English marine league is 3 nautical miles.