CHAPTER. VI. Of latitude and longitude. Division of the earth into zones and climates. 1. The arc of the meridian contained between the zenith of a place and the equator, is called the latitude of the place. The latitude of a place, on the terrestrial globe, is measured on the brass meridian, from the equator towards the north or south pole. If the place lies in the northern hemisphere, it is said to have north latitude; and if it lies in the southern hemis phere, it is said to have south latitude: so that the latitude of any place will be greater or less, according as it is farther from, or nearer to the equator. Hence the latitude under the equator is nothing, and the latitude increases gradually as we advance towards either of the poles, where it is 90 degrees, or the greatest possible; as will appear evident from consulting the globe. It is also obvious that a great number of places may have the same latitude; for if a circle be supposed to be drawn through any point of the meridian, parallel to the equator, all the places which lie under that parallel will be equally distant from the equator, and consequently must have the same latitude. 2. The angular distance of a place, on the surface of the earth, measured upon a secondary to the 'equator, is called the reduced latitude of the place. Secondaries to a great circle are great circles, which pass through its poles, and which are perpendicular to that great circle: thus, the meridians are secondaries to the equator or equinoctial. It may be also observed, that the true latitude of a place, as defined in the preceding article, would be equal to the reduced latitude of the same place, if the figure of the earth were truly spherical. The difference between the true and reduced latitudes shall be pointed out, when we come to consider the true figure of the earth. 3. Longitude of a place is its distance east or west from the first meridian, reckoned in degrees, minutes, &c. upon the equator. The choice of a first meridian has been a matter of considerable embarrassment both to astronomers and geographers, and even yet they are not perfectly agreed in their determination. The French formerly made their first meridian pass through the island of Ferro, one of the Canaries; and the Dutch fixed upon another of those islands, called Teneriffe, as the proper situation for this purpose. But the English, as has been already observed, make their first meridian pass over London, or rather over Greenwich, on account of the observatory being at that place. This disagreement amongst astronomers of different nations is not, however, to be considered as a matter of much importance; for whichever is regarded as the first meridian, the rest may be easily deduced from it, by noting the different points where they intersect the equator, and finding the difference. Though it is very natural to suppose, that astronomers and geographers would assume as the first meridian that which passes through the metropolis of their own country; the Americans, on account of having no public observatory, are still under the necessity of referring the longitudes of places to the meridian of London. 4. It is proper to observe, that the greatest longitude a place can have, is 180 degrees; and that the longitude of any place lying under the first meridian will be nothing. A great number of places may also have the same longitude; for if a meridian be supposed to be drawn through any point upon the globe, all places lying under that meridian, when referred to the equator, will be at an equal distance from the first meridian, and consequently their longitude must be the same. 5. The declination of a heavenly body is its distance north or south of the equator, or equinoctial, reckoned in degrees and minutes, upon a secondary to it drawn through the body. Hence, the declination of a celestial body is similar to the latitude of a place on the terrestrial globe; and the greatest declination the sun can have, north or south, is 23° 28′. 6. A division of the earth contained between two parallels of latitude, is called a zone. 7. The two tropics and two polar circles divide the globe into five zones. 8. That part of the earth contained between the tropics, is called the torrid zone. The breadth of this zone is therefore. equal to twice the greatest declination of the sun, or obliquity of the ecliptic, equal to 46° 56', or twice 23° 28′. The torrid zone experiences only two seasons, the one dry, the other rainy. The former is looked upon as the summer, the latter as the winter of these climates; but they are in direct opposition to the celestial winter and summer, for the rain always accompanies the sun, so that, when that luminary is in the northern signs, the countries to the north of the equator have their rainy season. It appears that the presence of the sun in the zenith of a country, continually heats and rarifies its atmosphere. The equilibrium is every moment subverted, the cold air of countries nearer the poles is incessantly attracted, it condenses the vapours suspended in the atmosphere, and thus occasions almost continual rains. The countries of the torrid zone, where no vapours rise into the air, are never visited by the rainy season. Local circumstances, particularly high chains of mountains, which either arrest or alter the course of the monsoons and winds, exercise such influence over the physical seasons of the torrid zone, that frequently an interval of not more than several leagues separates summer from winter. In other places there are two rainy seasons, which are distinguished by the names of great and little. The heatis almost always the same within 10 or 15 degrees of the equator; but towards the tropics, we feel a difference between the temperature which prevails at the moment the sun is in the zenith, and that which obtain when in the opposite solstice, the solar rays falling under an angle of more than 47 degrees. We may, therefore, with Polybius, divide the torrid zone into three others. The equatorial zone, properly so called, is temperate, compared with the zone of the tropic of Cancer, composed of the hottest and least habitable egions of the earth. The greatest natural heat ever observed, which is 35 degrees of Reaumer, or 111 degrees Fahrenheit, has been at Bagdad, at 33° of latitude. The zone of the tropic of Capricorn contains but little land; but it appears to experience momentary heats of extreme intensity. Most of the ancients, disregarding the observations of Polybius, conceived that the heat continued to increase from the tropic towards the equator. Hence they concluded that the middle of the zone was uninhabitable. It is now ascertained that many circumstances combine to establish even there a temperature that is supportable. The clouds; the greatrains; the nights naturally very cool, their duration being equal to that of the days; a strong evaporation; the vast expanse of the sea; the proximity of very high mountains, covered with perpetual snow; the trade winds, and the periodical inundations, equally contribute to diminish the heat. This is the reason why, in the torrid zones, we meet with all kinds of climates. The plains are burnt up by the heat of the sun. All the eastern coasts of the great continents, fanned by the -trade winds, enjoy a mild temperature. The elevated districts are even cold; the valley of Quito is always green; and perhaps the interior of Africa contains more than one region which nature has gifted with the same privilege. Nothing equals the majestic beauty of the summer in the torrid zone. The sun rises vertically; it traverses in an instant the burning clouds of the east, and fills the heavens with a light whose effulgent splendour is unobscured by a single shade. The moon shines here with a more brilliant lustre, Venus blazes with purer and more vivid rays, and the milky way glitters with augmented brightness. To this magnificence of the heavens, we must add, the serenity of the air, the smoothness of the waves, the luxuriance of vegetation, the gigantic forms of plants and animals, all nature more grand, more animated, and yet less inconstant and less changeable. 9. Those parts of the earth contained between the tropics and polar circles, are called the two temperate zones: each of which is, therefore, 43° 4′ broad. The north temperate zone extends from the tropic of Cancer to the arctic circle; and the south temperate zone from the tropic of Capricorn to the antarctic circle. The temperate zones enjoy the mild and varied charms of the latitude of a place on the terrestrial globe; and the greatest declination the sun can have, north or south, is 23° 28′. 6. A division of the earth contained between two parallels of latitude, is called a zone. 7. The two tropics and two polar circles divide the globe into five zones. 8. That part of the earth contained between the tropics, is called the torrid zone. The breadth of this zone is therefore. equal to twice the greatest declination of the sun, or obliquity of the ecliptic, equal to 46° 56', or twice 23° 28′. The torrid zone experiences only two seasons, the one dry, the other rainy. The former is looked upon as the summer, the latter as the winter of these climates; but they are in direct opposition to the celestial winter and summer, for the rain always accompanies the sun, so that, when that luminary is in the northern signs, the countries to the north of the equator have their rainy season. It appears that the presence of the sun in the zenith of a country, continually heats and rarifies its atmosphere. The equilibrium is every moment subverted, the cold air of countries nearer the poles is incessantly attracted, it condenses the vapours suspended in the atmosphere, and thus occasions almost continual rains. The countries of the torrid zone, where no vapours rise into the air, are never visited by the rainy season. Local circumstances, particularly high chains of mountains, which either arrest or alter the course of the monsoons and winds, exercise such influence over the physical seasons of the torrid zone, that frequently an interval of not more than several leagues separates summer from winter. In other places there are two rainy seasons, which are distinguished by the names of great and little. The heatis almost always the same within 10 or 15 degrees of the equator; but towards the tropics, we feel a difference between the temperature which prevails at the moment the sun is in the zenith, and that which obtains, when in the opposite solstice, the solar rays falling under an angle of more than 47 degrees. We may, therefore, with Polybius, divide the torrid zone into three others. The equatorial zone, properly so called, is temperate, compared with the zone of the tropic of Cancer, composed of the hottest and least habitable |